Marimo Moss Ball FAQs

By | November 4, 2018

This page contains common questions about marimo moss balls, including marimo care, facts, sizing, color, and more.

These questions are common ones we get sent from readers (like you!) via email and contains our replies to them so everyone can benefit from this information.

This FAQ is constantly updated as we get more questions. If you have one to ask, just leave a comment here or contact us and ask your question!

All of the following questions are real questions from real readers.

Last updated: 2/26/21.

Seachem Prime and Marimos

Question: We have hard water here in Dallas, and you mentioned Prime by Seachem.  My Marimo is in a glass container that only has maybe 1-1.5 gallons of water (including the original water the moss ball was living in).

How much Prime would I use for a small batch of water like what I have?  I bought a dechlorinator and put a drop in the water and swished it around.  Prime would take care of the dechlorination and eliminate the water hardness?

Answer: Prime is definitely my go-to choice for dechlorinating water and I’d recommend it for your marimo even in such a small container.

For your case, I’d just do a single drop into the glass container and let it diffuse by itself. That’s more than enough to dechlorinate even a container with a volume doubly yours. You seem to already have it down!

As for the water hardness, Prime doesn’t really remove minerals from the water (especially calcium), but more so used for removing chlorine, chloramine, heavy metals, etc. The solution will make your water safe for fish/marimos, but won’t reduce your overall hardness. It’s kind of confusing, but the easiest way I can put it is that it’ll take out all the dangerous stuff, but not the inert, neutral stuff. And minerals are often inert and neutral, so they stay in the water, which keeps your water hard.

There are other solutions on the market that lower water hardness and much more expensive solutions (such as using an RO system, distilled water, or remineralizing water (using pure water and adding minerals to “taste”), but for the casual marimo keeper, those are overkill!

If your goal is to lower your hardness, Prime isn’t the most effective option. But if you just want to make your water safe for marimos, Prime will do wonders.

Water hardness won’t really harm your marimos, and some mineral content is beneficial. But if you see white streaks, that’s a sign of too many minerals and that’s when you should worry. I’ve used it with live fish in a 10-gallon tank and two drops would work just fine- this stuff is super concentrated and is safe even for “overdosing.”

Can moss balls survive in a closed container?

Question: Hi, I recently got three marimo balls and they are in a glass jar with a cork top. I have read many articles to find the answer to my question and I haven’t yet. Can I keep the cork lid on the jar or does it need to be open for air to circulate through the jar? Thank you for your help and I look forward to your response.

Answer: You’ll want to keep the cork lid loose or use a permeable one to allow air flow. Marimo moss balls won’t stay green for long without a source of oxygen. Since light hits the jar, they’ll undergo photosynthesis (they’re really just algae balls) and this process requires a constant air supply in order for them survive. Without proper air supply, they’ll likely brown in a few weeks when all the air has been used up in the jar since it’s pretty much a closed system.

How fast do moss balls grow?

Question: How fast or slow do these grow?  Did I miss that info on your page?

Answer: They grow pretty slow- actually, really slow. About an inch per 5-8 years.

Moldy moss balls

Question: I just got mine yesterday. It turned dark green when I washed it then put it in my ten-gallon tank. I have 3 small bettas and two platies. This morning it has turned a yellowish olive green kinda brownish in spots. I took it out in put in a stainless steel bowl with cool water and a tiny bit of sea salt. I washed it out again then put it back in the tank. It still looks like a moldy kind of color

Answer: It’s probably stressed from the shipping and the changing water conditions. It’ll need some time (like 2-3 weeks) for it to adapt to new water parameters. Browning, stringy, and dark-colored algae are all completely normal when you first adapt them to your water!

I’d suggest leaving it in the aquarium with indirect light and letting it sit for a few weeks. If you have plant fertilizer, just a small dose will help tremendously. Marimos are really just spheres of algae, which act and behave just like any other aquatic plant.

Aquarium plants all have a period where they adjust to new water parameters and change how they grow by killing off old growth and growing new algae that are suited for their environment.

Betta fish and marimos

Question: Hi guys!  My plan is to add small ones to my small aquarium with a fish.  Are these Marimo free of bacteria or parasites that could attack my Betta?

Answer: They’re free from parasites, bacteria, and other nasties as they come from natural sources and then quarantined afterward for a week.

However, nothing is guaranteed in the aquarium world. You’d be surprised at home many major pet stores label their plants as “snail free” only to have baby snails surface on the glass two weeks later!

If you’re paranoid about hitchhikers getting into your betta’s aquarium, you can quarantine them. Just get a small container with some clean water and add a drop of plant fert if you have some.

Leave it somewhere with indirect sunlight for about two weeks and check for snails, oily water, and browning marimos. If everything looks good, they’re ready to go! This will also give them some time to recover from the shipment.

Marimo and calcium buildup

Question: Hi! I have a couple of questions about Marimo care.

Firstly, in your care section, you mention leaving the container open to the air, with no lid. However, most of the pictures you have up show the containers with lids. What’s up with that?

Secondly, my marimos have tiny hard, white bumps on their strands. Is this what they are supposed to look like?

Answer: Good eye! Thanks for pointing that out. Marimos can be stored safely in capped container temporarily, but eventually, need access to available oxygen/nitrogen in the air.

This is important for photosynthesis, as marimos are really just balls of algae clumped together! If they’re stored long-term in a closed system, they’ll start to rot and break down. The images are sourced from around the web and not necessarily our own pictures. They’re licensed for commercial use, so we picked out a few that we found showed off the marimo in its glory!

As for your question, it’s likely due to hard water buildup. It’s probably calcium and a mix of other minerals that form over time from water, or even in the lake if the marimo isn’t free to move around.

This can be rectified by simply cleaning them using a pair of tweezers and picking those strands off. If the issue persists, consider using R/O or distilled water, or doing more water changes. I don’t know what setup you have, so I’m just throwing some broad suggestions out there. It depends on your setup.

Lid or no lid?

Question: I change the water weekly since I change it that often is it ok to have a lid on, or should I opt for no lid?

As for the buildup, I have been using spring water because the tap here is basically pool water. I guess the mineral content is too high? Are the calcium deposits harmful to the Marimos? I will try using distilled and see if that helps 🙂

Answer: If you change the water weekly, that should be enough to get fresh O2 into the habitat from the water and from merely exposing the tank to the atmosphere. So I think you’ll be fine there. Just be careful of temp swings- the hotter it gets, the more difficult it gets to absorb O2 in the water. If you see condensation, you should pop the top.

Yeah, the tap water will kill them over time. Treated municipal water will do a lot of harm, so good on you for the spring water. But even then, it contains natural minerals (and sometimes added minerals) which may cause buildup.

You can try a 50/50 mixture of distilled and spring water on your next water change and see how it goes from there. The calcium deposits shouldn’t really affect them other than cosmetically- as long as they’re not like engulfed in calcium, they should be okay.

Moss ball shedding

Question: Hi, for a long time now, since I got my moss ball in May, it seems like it is shedding.

Whenever I go to change the water every two weeks, there are little brown flakes everywhere in the water, and a lot of them. Is this normal for a marimo ball? I also am wondering if my marimo ball needs a current? I have mine sitting in stagnant water.

Answer: Is your marimo housed by itself? Or are there other inhabitants in the habitat with it?

And by water, do you mean you’re doing a water change to the container? Or is it sitting in a pool of water and you’re adding to it? Is it submerged or just partially submerged?

I’m assuming you have it sitting in a container with water and you’re adding to it every other week. If this is the case, the brown flakes could be algae growth that’s slowly growing on the marimo and the water you add flushes them off. This isn’t necessarily alarming, as algae and other beneficial bacteria will grow in any pool of water over time, but excessive lighting from indoor lights or the sun speeds up the process.

Also, the stagnant water definitely is a factor if the above is correct. No water movement means it’s a lot easier for bacteria to grow in place. Adding a small bubbler will do wonders and give it a nice ripple effect- though this may not be applicable to your specific setup! In nature, they’re found on lakebeds rolling around at the bottom gathering moss, pretty much. so there’s always a small current pushing them around as they “snowball” more algae and grow.

In your case, I’d first try moving it somewhere with slightly less light. If that doesn’t work or if it’s not possible, then adding a small bubbler should do the trick. They’re cheap and can be bought for $6 shipped online. And they barely use any power even running 24/7- probably just a few cents per year.

Do marimo balls need food?

Question: I have a happy little moss ball that shares my office. According to your website’s descriptions, it seems to be doing fine and has actually grown a little. I have had it for about 10 months.

My question: do marimos need food? I’ve seen this food on Amazon, and wonder if it is a scam. If marimos do need to be fed, what do you suggest?


Answer: To keep it simple- they don’t need food necessarily as algae will grow with just light =]. If you want them to be vibrant in color, you could try adding a dose of plant fertilizer for aquariums. But they should do fine with just sunlight and regular water changes.

Let me know if you have any other questions.


Do marimos provide enough oxygen for betta fish?

Question: If I have 3 Marimos in a 1-2 gallon aquarium with one Beta would the Marimos produce sufficient dissolved oxygen for the Beta to survive comfortably?

Also, what is the stick-like brown branch that is often seen in pictures with the Marimos?

Many thanks!

Answer: Marimo balls will likely produce minimal free oxygen for a live betta fish in a small aquarium. I would tell you to look into using a bubbler, but I believe betta fish prefer stagnant water over turbulent currents- as they’re natively found in rice paddy fields which have no current.

If you’re worried about getting enough O2 for the fish, you could always use a small bubbler with a softener to minimize the bubble splashing. This would circulate the water and provide more free O2 and nutrients for the fish and the marimos (they’re basically algae which will need free nutrients to grow).

The brown branch you see is often a thin driftwood variant. You can easily replicate the same effect by finding a twig or stick, boiling it to kill free parasites, then waterlogging it so it sinks! This will save you some cash if you don’t want to spend on driftwood since it’s relatively expensive.

Let me know if you have any other questions about marimos!

Green slime and marimos

Question: My teacher has two Marimo Balls and they have seem to grown a very large thick layer of slime stuff (about an inch thick) over the both of them binding them together. I pulled the slime layer off, but some of it is literally bound to the ball.

They also look very dark green and have some spots of brown and white dots. Long story short they look very unhealthy and gross. I was just wondering if you had any tips on how to get them looking normal and healthy again.


Answer: It sounds like you’re having a case of “green slime” as they call it in the aquarium hobbyist world. This stuff is very thick, slimy, and sticks coherently as one fabric of slime that’s a pain to get rid of. It may also have an odor and forms across the top of the water surface when given enough time.

This is usually caused by overfeeding, too much lighting, or poor water conditions that allow algae to flourish. If there’s also too much plant fertilizer being used, that could also cause the slime to form.

I’d start by minimizing feeding (if applicable) or moving the container away from bright lighting (if it’s by a window or under a light). If plant fert is being used, cut the doses in half. And water can be partially changed (20-30%) once a week.

Lastly, the slime will have to manually be removed by hand or aquarium tongs. Remove it whenever you see it and keep up the maintenance. If you leave it there, it’ll prosper and just reform over time. Be diligent and remove it at first sight!

Do all this for two weeks and it should get rid of the green slime.

Let me know if you have any other questions.


Rolled marimos – fake or real?

Question: I ordered a Marimo from a different site (I had wanted to order them from you but my debit card wouldn’t work) but today I found out they are artificially rolled from filaments from Lake Akan.  Is this a common practice since Marimos can’t be removed from their natural lakes? Their contact page is defunct and y’all seem trustworthy. I’m just curious if this is how most marimos are sourced.

Answer: Sadly, that’s the nature of a lot of marimos nowadays.

In nature, they form on the bottom of lakebeds by rolling up and “snowballing” themselves into spheres over the course of many years.

When they’re rolled, they basically use the “scraps” of other marimos and just mush it into a spherical ball. This is obviously an unnatural process and is like trying to force a marimo into existence.

Some lakes prohibit removal, but I’m not 100% clear on the laws. It’s possible that they couldn’t remove it from the lake and had to roll them artificially, but there’s no proof of that as far as I know.

Please let me know if you have any other questions.

Marimos and Betta fish – Temperature differences

Question: How does keeping a betta and a moss ball work if a betta needs a heater and water temp from 72 to 80 and a moss ball needs a much colder water temp? I’ve been thinking about setting a ten-gallon tank and getting some live plants and a betta and a moss ball and a lot of people keep moss balls with bettas. How does that work if they both need such different water temps?

Answer: The best way to go about this is to use ambient temperatures. You’re correct. Moss balls thrive in colder temperatures, but that doesn’t mean they won’t grow in warmer ones.

You won’t necessarily harm them, but they just won’t grow as fast. However, their color, density, and shape will remain the same and temperatures don’t really matter. The only thing temps really affect is the growth rate and possibly algae blooms (which occur in warmer temperatures).

Other than that, it’s not too much of concern.

However, one trick you can do is get a mini heater. Depending on the size of your tank, you could place the heater at one end (the warm end) and have the marimos at the cool end. Since the heater isn’t powerful enough to heat the whole tank, only a portion of it will be heated and the betta will hang out there if it wants to.

As for the plants, you can research them and place them wherever accordingly.

This would provide a multi-temp environment within a single tank, and works perfectly depending on the size of the tank and the wattage of the heater.

Reshaping damaged marimos

Question: My moss ball is ripped at the top and is now in a large sausage shape should I leave it, rip it or mold it back into shape?

Answer: You can safely mold it back into shape. It should be no problem!

Addendum: Marimos can be molded into pretty much any shape. They’re connected because of fibers that the algae use to connect to other algae fibers and form a ball. They can be molded into anything as long as the strands together- so you could actually have spherical, square, triangular, or whatever other shapes you can craft.

Marimo won’t sink

Question: There is sand in the moss ball and I squeeze it and it will not sink, I do not know what to do.

Answer: The best way to get a moss ball to sink is to simply let it float in the water and leave it alone. Over time, it’ll slowly soak up water and when it becomes waterlogged, it’ll slowly sink.

Squeezing it during this process will just drain the water out and restart the process. The sand should be cleansed by washing the moss ball. But if you want it to sink, after cleansing, let it float around. You can speed up the process by putting the ball near some turbulent water, such as near an air bubbler. The bubbles will help get the marimo to sink.

Moss ball flattening kits and carpet effects

Question: Hi I have Marimo Moss in a planted tank and it keeps trying to tangle on the leaves and roots.

Can this be dangerous for my other plants? Also, I’ve seen kits to turn moss into flatter surfaces to make “carpet” or “mats” for fish to lay on and want to know if this is ok.

My moss ball grew and split and right now is doing well taken apart into really small pieces stuck together looking a bit like carpet but I don’t want to hurt it using the kit if it’s an issue. I’d like to create a carpet effect for my fish and possibly a little soft cave or something. So many of the fish caves have sharp or rough edges and surfaces and I know that’s really bad for especially my Betta.

Answer: Marimos are basically strands of algae that have scrambled together to form a spherical shape after rolling around on lakebeds for many years. This shouldn’t pose any danger to your other plants unless they’re sensitive or easily uprooted (for rooted plants). I’m not exactly sure what other plants are in your aquarium, but the worst that could happen is that they’ll somewhat “eat” the moss ball, which may harm the marimo because it won’t have access to light nor nutrients.

As for the flattening kits, this will ruin the structure of the moss ball. Once you flatten them, you won’t be able to get the natural structural integrity of the moss ball prior to flattening.

Once you smash them, you can definitely roll them back into a sphere, but they won’t be as durable and won’t maintain their natural shape as before. This really depends on your goal with the moss ball- if you plan to use it as a carpet plant, I don’t see any problem. But if you want to ever roll it back up again, it’ll likely have an oblong shape even after shaping it again.

As for carpeting plants, there are a lot of awesome aquatic plants that can be used for carpet effects. I highly recommend java moss and dwarf baby tears as starters. And you’re totally right about the sharp edges for decor- definitely avoid this or cover it with rocks, plants, or smooth it out by sanding the edge.

Keeping moss balls out of water

Question: Can I keep a marimo moss ball out of water? Will it live? Thanks.

Answer: Unfortunately, marimos will dry up over time. They can survive out of water for an extended period of time, but they’ll eventually start to shrivel if you don’t give them a supply of water. Just think of them as another houseplant. They’ll need to be spritzed or “dunked” once every so often to keep them green and alive.

Marimos and customs

Question: I will be traveling to Japan this summer. I am wondering if you know if marimo can be brought back on a plane through customs?

Answer: Bringing them into the US has been no issue from personal experience so far. However, when you’re actually flying them back and carrying them personally, I think it’s more of an issue with airport security rather than US customs. I couldn’t any strict laws that prohibit marimo moss balls into the US, but I couldn’t find any approval either. I’d definitely give the airport a call that you plan to fly through and see what they say for a detailed response.

Temperatures – Bettas and marimos

Question: I just bought a beautiful Marimo ball for my betta tank.  It is generally heated for the betta.  Will it help the longevity of my Marimo to take it out of the tank for most of the night in cooler water and place it in the tank for a couple of hours?  Don’t want it to die from the warm temps.  Would this help?

Answer: Depending on the size of your tank, you may be able to set up a temperature gradient for your betta and your marimo. You can place the heater on one side, and place the marimo on the opposite. This will create a gradient effect within your tank and allow you to have both at the same time.

If your tank is small, you could try using a smaller heater that’s rated for a lower wattage. This will output less heat and will give enough just for your betta to hang out in a warm spot while having the rest of the tank being cooler. Conversely, you can upgrade your tank size to create this gradient.
If neither option works, you could try rotating the marimo out of the tank every soft often. Definitely doesn’t have to be nightly, unless you’re willing to go through the trouble. you could rotate weekly with weekdays in the tank and weekends out.

Betta fish and marimos in the same tank

Question: I want to purchase some moss balls from your store since after reading I am not quite sure I trust the ones at the pet shops or some I found on Amazon. I just have a few questions, I read that it is great to keep in a tank with a betta fish. I currently have two moss balls in my 5.5-gallon aquarium with my better fish.

However, being that betta like warm water between 78-80 degrees Fahrenheit I find it contradicting to what your article says marimos need to thrive.

You say that they need cool water but bettas live in warm water. I just took my marimos out of the tank, gave them a good clean and put them in the fridge for a few hours. I am going to put them back in the tank but before I order more I just want to clarify that I do have them living in my betta tank which is kept at about 80 degrees.

Answer: This is a question that comes up a lot from our readers.
As any betta fish owner would know, they prefer slightly warmer water than room temperature, as you stated around 78-80F are common temps for betta fish.
Marimo moss balls are naturally found in much colder temperatures in their native habit (at the bottom of a cool lakebed). Provided that the temps do fluctuate in the wild, marimos can tolerate quite a bit of temperature changes.
Ideally, they prefer much colder temperatures, but not every aquarium can provide this range.
Thus, it’s impractical for most marimo and betta owners to have an aquarium that can house both a betta fish and a marimo moss ball at the same time- while keeping both housed in ideal conditions.
There will definitely be some kind of give-or-take compensation between the two species. Since the betta fish is the more sensitive organism, it only makes sense to be partial towards the betta fish. Marimos will do OK in warmer conditions, but they won’t flourish.
But then again, remember that your moss balls can survive even in less-than-ideal conditions. This won’t necessarily harm them, they’ll just grow slower and also may not be able to have that lush green coloration.
You may be able to supplement and correct this by dosing with a weekly plant fertilizer, or if your betta provides enough waste as nutrients for the moss balls, they may do OK also.
The last thing that I suggest would be to set up a temperature gradient for the tank. Place the moss balls at the cold end and a small tank heater at the warm end. The betta can then swim around near the heater as needed while the moss ball gets the colder water.
This only works for larger tanks with weaker heaters that don’t heat the whole thing, so this may or may not work for you.
Since you have a smaller tank, this probably wouldn’t be practical for you and we’d suggest just going with the regular dosings. Moss balls eat up “waste” in the tank and just need a source of nutrients and light to flourish.
If you have any other questions, let us know and we’ll be glad to help!

Do marimo moss balls need plant food or fertilizer?

To keep it simple, marimos only really need 3 things to stay healthy:
1. Light
2. Water
3. Nutrients in the water column
If you have fish, shrimp, reptiles, or any other invert that produces waste in the water column, no additional plant food/fert is required! The marimos should be OK with the natural waste produced by the inhabitants.
Light can come from indirect sunlight or an artificial LED light.
However, if you have marimos in the same tank w/ other plants, adding plant food/fert would be beneficial for all the plants in the aquarium- as they’ll each need some nutrients to stay lush and green.

Infested moss balls

Question: I would really like to own Marimos but am worried about infestations. I am mostly concerned about worm infestations in the Marimos since I’ve read many reviews from one particular sellers site about the balls being infested with the eggs of worms, snails, and bacteria and just falling apart and smelling fowl. Is it normal for the Marimos to have parasites and can I treat them with a deworming agent prior to putting them in their future home?

Answer: Infestations is definitely something that happens, whether you purchase the balls direct from the source (i.e. the lake), or you get imported. In fact, the more “natural” the ball is, the more chance of it having some kind of worm or parasite. After all, these fibers rolling around on a lakebed brimmed with aquatic life will have trapped and become home to many different species.
The fix is simple- you can leave the marimos soaking in a minimal saline solution overnight for the first 24 hours. Then you can use (very) diluted dewormer or parasite killer on the second night. On the final night, you can just soak the solution again in salt water to get rid of any remaining critters. You’ll want to squeeze out the ball each night before placing into the new solution. This will help drain out the life on the ball.
By the third night, you should have a relatively clean marimo. You’ll then want to let it sit (out of water) in direct sunlight for at least a day to kill out the rest of the aquatic life. The sun and lack of water will finish up the rest. Only aerobic parasites and worms will still be able to survive, but the second night should have cleaned them up. You can repeat the process again if you wish, but this is generally what we do for imported marimos and we’ve rarely had any mention of parasites or worms after this process.

Distilled water and marimos

Question: Hi! I’m completely new to taking care of moss balls. I’ve put them into a glass jar (open top) by themselves, no fish or anything.
We have pretty hard water in MN, is it ok to use just distilled water?To keep the water cool, is it ok to swirl an ice cube around into their water?


How clear does their water normally stay? When I first got them I put them in our hard water (bcs had no distilled) and for about a week they looked ok but there’s tiny fluffs of green all along the sides of the container. I just swapped the water out for distilled yesterday and cleaned out the jar. Are the green fluffs normal?

Last question, I promise! Do you know of any other water plants that do well in the same environment as moss balls?
Thanks so much!

Answer: Marimos can survive in distilled water, but just not forever. The problem with distilled water is because it’s too clean. There are no minerals, trace elements, or other sources for bacteria to grow and flourish. Of course, the bacteria and spores floating in the air will eventually make their way into your open-lid jar, so that’s OK. But using either purified water (treated with water conditioner) would be best because it leaves useful minerals behind. However, if you don’t have water conditioner, you can use distilled water and add a few fish flakes in there just to jumpstart a new bacterial colony in the jar!
As for keeping the water cold, an ice cube is fine. Marimos can tolerate hot temperatures, just not extended (as in weeks). A few days here and there is OK, especially if temperatures fall during the night. But if you’re afraid, adding ice cubes is no problem. It won’t “shock” them or anything.
Their water will stay clean for the most part. Over time, if you don’t have fish or some other amphibian to eats algae, the algae will bloom. If you have the jar under sunlight, bright lights, or any other light, this will make the algae grow even faster. The “green fluffs” you mentioned around the sides of the container are likely algae/diatoms. They start to establish and will take over the jar if there’s no fish in there that eats them.
Marimos are algae balls and will give rise to other algae around the jar. It’s difficult to control algae if there are no other fish to eat it. The best way is to minimize light and do regular water changes. There’s nothing wrong with algae, but they will eventually grown so much until they block your vision into the jar!
As for other plants that are compatible with marimos, this depends on your water conditions and temperature. Some easy ones I can suggest that pretty much in any condition are java moss, java fern, or duckweed (floating plant).
Please let me know if you have any other questions =]!

How much water do marimos need?

Question: I loved your article. But no one ever says anything about the water requirement.  I don’t want to make assumptions, so if you can give me an idea that would be grand.

I have three marimos about 1 1/2 inches wide.  They are in about 1+ gallon of water.

They’ve been with me for about 2 weeks and they look very healthy, bubbly, and floating and falling.

Answer: Your current setup of 3 marimos in 1g of water is perfectly fine. They’re very adaptable to a variety of conditions- everything from huge 100 gallon aquariums to 14 ounce mason jars. More water offers a more “stable” environment whereas less water tends to have more fluctuations.


In other words, the more water you have, the harder it is to affect parameters like pH, KH, TDS, and other various factors. This is nice for fish and other aquatic inverts that are in your aquarium!

Marimos and cats

Question: Hello, thank you for your informative guide. My husband is worried about keeping them in open containers because the cat drinks the water! He is worried it might hurt her. Do you have any information on this?
Thank you.
Answer: If your cat seems to be interested in “fishing” the marimo out or sipping out the aquarium water, you may consider relocating the setup just to prevent any possibility of a cat attack.


Since the marimo doesn’t really move (unless it decides to float), it shouldn’t attract too much attention.
But if it does, I’d suggest to put the container in a more secure place- just to be sure! If the cat attempts to drink the water, you can always use an aquarium topper to cover it. Or if it’s a mason jar, you can use a jar lid and poke holes in it.
The moss balls don’t need huge amounts of air exchange- even one or two holes is plenty!

What decorations are safe for marimos? How about paints?

Question: I’m looking to purchase some hand-made ceramics from local artisans and I’m not sure if they will be toxic to the water. I understand you don’t know exactly what they are using in their craft, but I was wondering if you knew of anything to look out for with your experience growing these little guys. Should I stay away from ceramics? Certain types of paint?

Answer: First, local crafts should be safe for marimos (but NOT fish) for the majority of common materials they’re made from- terra cotta, plastic, glazed ceramics, porcelain, brick, earthenware, etc.

The main concern is if the piece leeches a considerable amount of residue into the volume of water. You can usually purge most of these compounds out by soaking it in a container for a few days and emptying out the water frequently. The first round will eliminate most of the loose residuals and you’ll be able to see the water discolor or bits floating around.

Even if you didn’t do this, your marimo tank will eventually absorb all the material from natural diffusion as long as you do regular partial water changes. Paints are also safe as long as they’re waterproof and FULLY CURED. When the paint becomes dry and cured, you can test it by soaking it in water and seeing if any leaching occurs. Most aquarium safe paints will be labeled as so.

If the piece is a ceramic that’s been glazed or coated with something that gives you that “smooth” texture, it’s likely water safe. Again, always test first before adding any live organisms.
However, this only goes for marimos themselves. If you plan to add inverts, you’ll want to make sure the material is 100% safe first. Marimos are much more hardy to toxins and can be easily cleaned by rinsing under water and giving them a squeeze. Plus, they show signs of “wilt” slowly over time which gives you a lot of room to address the problem. Whereas fish will be eliminated within hours from toxic compounds leaching into the water.

What’s the best aquarium size for moss balls?

Question: I also am trying to decide on the habitat they will be living in. I see these little jars online that people pack the Marimo’s into.. That can’t be good for them can it? I know at the end of the day, it is algae but I see them as actual pets lol. What do you recommend as a good amount of space for 1 Marimo?

Answer: This actually gets asked a lot around the community.

In nature, they are found on lake beds rolling around at the bottom and snowballing algae which gives them size over many decades. They get wedged between rocks, stuck in cracks, and covered with debris and leaf litter. Yet they do fine, albeit they grow slower.

Even if you stuff it into a tiny 2×2 cube, it SHOULD do okay as long as sufficient sunlight, debris (food), and air exchange is provided. Though it’s obviously not going to be ideal for the best growth. It’ll probably just be idling and remain the same size for years to come.

As with fish, the bigger the tank, the more favorable the environmental conditions become for them. I don’t have a minimum size to suggest, but I can say the largest container would work best with plenty of air circulation, light, and some kind of fish to produce waste.


Question: Hello, Ive noticed a thin film that builds up on the top of the water in each of my Marimos habitats, kind of like a thin skin. It comes off easily if I drag a spoon thru the top of the water.

Im changing water once a week but have to skim a couple times a week. Any ideas, is this normal? I havent come across this in any reading Ive done.


Answer: It’s likely biofilm. It’s actually a very fine form of algae that forms an oily surface across the water.

It’s usually due to any of these conditions:

  • -too much light/not enough nutrients
  • -too little light/too many nutrients
  • -poor water circulation

If you’re dosing with a plant fert, I suggest cutting dosings in half. If you’re using a light, leave it off for a period of 48 hours and see if it improves. If you have no water flow, add a powerhead or air stone to improve circulation. The tank’s surface should have ripples all across the surface and not in just one area.

Another trick you can do to remove it quickly as a temporary solution is to use a dry paper towel and drag it across the surface.
After doing these practices, it should improve rather quickly!
Let me know if you have any other questions.

How to make marimos float during the day?

Question: Thanks for the info! The moss balls arrived today and I just was wondering….I have read about them alternating between sinking and floating as their photosynthesis needs dictate…do you expect that these I have received will do that? I have a larger container picked out for them to highlight this activity (hopefully!). Thanks for your time and information. 

Answer: The floating/sinking buoyancy cycle is indeed due to photosynthesis (a relatively recent discovery in the community). The bubbles that form inside the balls during the day makes them float up large vertical containers until the light goes away- to which then they release the bubbles and sink back down.

Some marimos will exhibit this, but not all. I’ve yet to see a marimo that repeats this behavior consistently as it’s almost seemingly “whenever it feels like it.” I think it’s because of the daylight photoperiods and how no two days are alike from changing patterns of weather. By nature, all marimos that were naturally harvested (not clumped together by human hands) are capable of doing this up and down cycle (and they do in the lake). But in the household, there are too many disturbances that diminish this effect.

From my experience:

  • If you provide enough direct light early in the morning, they have a higher chance of lifting up the water column.
  • If the light comes in the afternoon, they may not float or will “half-float” in the column.
  • If you can maintain the cycle of light (12h of bright light, 12h of darkness), they have a higher chance of showing the behavior on repeat.

But in the typical household, ambient light, artificial lighting, and other sources disturb this cycle so that’s probably why they don’t always float on queue- at least that is what I think.

Out in their natural environment, the cycle of sunlight is bright followed by pure darkness in the lake at night. Even though each day is different, the day/light cycles are to the extremes (very bright followed by very dark). In the household, there are too many elements that bleed light and disturb this. But if you can mimic their natural environment, you can probably make them float.

I hope this explains the phenomenon somewhat and helped you out. If you position their tank somewhere that has no other light during the night- and strong light during the early morning until sunset, you may be able to make them float. Try to eliminate other lights at night and keep it completely dark for 12 hours. Artificial lighting during the day with a grow light for 12 hours positioned directly over the tank may also work.

How to keep marimos in Mason jars

Question: My intention is to make them their own space in jars that have air flow. For example a flip top jar slightly ajar lid for oxygen but keeps cat out of it. For now I’ve put in  distilled water all together in one jar. Your info is mainly for aquariums.

Can I get the low down for non aquarium set up?  What kind of water is or is not okay? What do I need to supplement  the water with besides indirect light and air? What should I not use when adding things to the jar? Example; rocks, branches, figurines etc. Thank You!

Answer: Mason jars and marimos are a signature craft project and they should do well provided ample lighting and some fish flakes deposited into the jar at the beginning to “dirty” the water.

This is not needed if you place it near sunlight. The debris will allow beneficial bacteria to propagate and provide some delicious food for the algae to munch on.

They can tolerate a variety of conditions and should do fine in jars with some dirty water and ambient lighting. If you want to encourage the growth of bacteria for their food source, place the jar near direct sunlight.

But watch out for algae growing on the edges of the glass. Throw in a fish flake or two to help jump start the colony at the beginning.

Here are some other tips we’ve picked up:

  • Any conditioned water is acceptable (water from your sink + water conditioner).
  • Marimos don’t need any special equipment- they just need light and water for proper growth. If your container is large enough, you can add an air bubbler on the lowest setting to help “roll” them around, but it’s not a requirement.
  • The water doesn’t need any special supplements. You can add “general aquarium plant” fertilizer if you want to help enhance their color (we use Seachem Prime).
  • As for decoration, they do fine with anything. You can decorate it with rocks, sticks, figurines, etc. They’re not going to poke themselves on sharp objects or get caught like fish. Just make sure it’s aquarium-safe and doesn’t leech paint or chemicals into the water column.

16 thoughts on “Marimo Moss Ball FAQs

  1. Rudje

    I’m thinking about getting a marimo as a beginner aquarium plant, and I was wondering if it would be healthy for them to have something blocking light to the tank for part of the day? I live in a place that gets a lot of sun most of the year as well as with five people with erratic schedules, and I don’t want an excess or lack of light to stress out the pet orb(s). I know they’re native to latitudes much farther north than I am and I don’t want the light change to be too stressful.

    1. Anthony (BallsOfMoss) Post author

      Hey Rudje,

      Marimos are perfect beginner plants because they’re one of the lowest maintenance aquatic plants you could possibly get. Lighting, in my experience, seems to be one of the more flexible necessities to them as they adapt to various conditions and lighting frequencies. I’ve tried full spectrum lights for 12H, 4 on/4 off (supposedly helps with photosynthesis), and even just ambient lighting from the lamps in the house and I’ve experienced mixed results, with more light meaning healthier marimos- generally.

      If you’re afraid of blasting them with too much lux, you can definitely partially block the light or use something to diffuse it. I wouldn’t worry too much about the inconsistency in the lighting schedule as they grow very slowly and will definitely show signs of inadequate lighting and give you plenty of time to rectify the problem. If they start to turn into an unhealthy coloration, you can play around with the lighting schedule to fix it.

      Another solution is to simply place them away from outdoor lighting and just use an aquarium LED on a timer or manually turn it on/off daily. This is what I use for my setups and it works since there’s not much light where I set my marimos other than some table lamps which are only on during the night.

      Let me know if you have any other questions.


      Hi There:

      Could you tell me how many marimos would fit in a half-gallon container?

      Thanks much.

  2. Steven Frigault

    I have recently added two Marimo balls to my 30 Gallon tank. I have one Beta, several Glass Shrimp ands three bottom feeders as well as two snails. Within a few days I noticed a white substance on the Marimo Balls. Reading on line I think that it is an algae growth (on algae balls?). My water is very clear and well balanced. Any suggestions?

    1. Rob Post author

      Hey Steven,

      How’s the lighting situation in the tank and feeding schedule? What are your nitrite/nitrate readings?

      1. Nikki

        There is mold growing on the top of the water I keep my Marino in. Can I add anything to get rid of the mold?

  3. Sally

    I bought some marimos online and they arrived 3 days ago… I noticed that they seem to be rolled marimos and they’ve been shedding bits of algae fibers… is there a way to prevent them from doing so? I don’t want them to shrink or have bald spots on them…

  4. Anonymous

    Enjoyed reading the article above, actually explains everything in detail,
    the guide is extremely interesting and effective.
    Best regards,
    Demir Zacho

  5. Anonymous

    Thank you so much. At last, I found an article which I am
    searching for quite a long time. I hope that it will help me also.

    Best regards,
    Harrell Henneberg

  6. Rosalba Cowper

    Aw, this was an extremely nice post. Spending some time and actual effort to generate a really good article… but what can I say… I hesitate a whole lot and never seem to get nearly anything done.

  7. Rosie

    Heyy! will my moss ball go into shock if it is put into cold water to quickly? Also- there is a little brown spot on my little dude, is this normal?

  8. Leonardo

    You’ve probably heard of making terrarium with moss and ferns, succulent and cacti, and other easy-to-maintain plants, but have you heard of Marimo moss ball terrariums? Marimo (also known as “Japanese Moss Balls”), are a rare and unique species of algae native to Lake Akan in Hokkaido, Japan. The Marimo species is found only in a small number of lakes in the northern hemisphere. Join us to craft your very own whimsical yet practical aquatic ecosystem.

  9. Brittany

    I order two baby marimos balls off a different site. I didn’t realize how tiny they were going to be! They’re going to arrive in small jars which are cute, but I was wondering if I could put multiple balls into an aquarium? And which fish could I put in with them that could handle the colder and/or salt water and still be beneficial to my marimos? And If I order a bigger Marimo from your site could I house them together? Also Should I get a bubbler or any other accessories to help them thrive?

  10. Michael Borden

    I have four Marimo Moss balls that are nearly 1.6″ in diameter at this time. I live in Florida, and I breed the Mosquito Fish (FL native species) for personal aquariums, and for the biological control of mosquito pupae & larvae. I lightly flattened three of my moss balls in order to create an appearance of “the perfect” Mosquito Fish spawning bed .
    Can Ghost Shrimp or Cherry Shrimp also use the Moss balls as a protective spawning bed?
    Will I be able to keep my Ramshorn Snails with a moss ball, without the fear of the snails predation?

  11. Michael Borden

    I have four Marimo Moss balls that are nearly 1.6″ in diameter at this time. I live in Florida, and I breed the Mosquito Fish (FL native species) for personal aquariums, and for the biological control of mosquito pupae & larvae. I lightly flattened three of my moss balls in order to create an appearance of “the perfect” Mosquito Fish spawning bed .
    Can Ghost Shrimp or Cherry Shrimp also use the Moss balls as a protective spawning bed?
    Will I be able to keep my Ramshorn Snails with a moss ball, without the fear of the snails predation?


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