Aegagropila linnaei, better known to aquarists as Marimo or moss ball, is a type of algae that grows into green balls under certain conditions. They are popular as low-maintenance pets because of their interesting shape, bounciness and lack of need for special care, and can be kept in a bowl or vase and in some types of aquarium. Despite this, there are some things you can do to promote optimal growth and health of your Marimo!
A proper environment
Marimos don’t have many requirements when it comes to housing, but there are a few things to consider. First off, don’t keep your Marimo in a spot that gets a lot of sun. They naturally appear on the bottom of lakes where it’s quite dark, so they don’t react to over-exposure to direct sunlight well.
Another point to keep in mind is that, although some aquarium inhabitants like shrimp and dwarf crayfish love to forage on and hide behind a Marimo, there are also creatures that love it a bit too much and will attempt to eat or destroy it. Plecos aren’t good Marimo tankmates. Goldfish usually don’t make a good match for a Marimo either because they love to eat algae and bigger types of crayfish may also destroy it.
An interesting note about Marimo is that it can actually be kept in brackish water – Wikipedia states:
In Lake Akan the epilithic filament form of marimo grow thickest where dense salty water from natural springs flow into the lake.
Marimo is also noted on various aquarium plant websites to be able to thrive in waters with a salinity up to mid-end brackish (1.015 – check out the guest post I did for Sitting by the Koi Pond for more info on brackish aquariums) and adding some salt to the water is sometimes recommended when dealing with a Marimo that is turning brown.
Whether you keep your Marimo in a bowl by itself or in an aquarium, water changes are always important. During the summer, try to do a water change at least once a week – change all the water in a bowl, or around 25% in a filtered aquarium. Tap water is fine for this. This prevents dirt from accumulating on the Marimo.
A clean Marimo is a healthy Marimo! It’s a good idea to pick up your Marimo once in a while (especially if dirt particles have accumulated on it or if it has turned a bit brown or greyish) and gently wash it by squishing it a few times in a container with some clean water. You can then roll it around in your hands a bit to prevent it from falling apart from the squishing; this should be done very carefully. It helps the Marimo maintain its nice round shape, which it would naturally get from being rolled across the lake floor by the current. This unfortunately doesn’t happen in a bowl, vase or aquarium, so it’s a good idea to help it out a bit once in a while by re-rolling it yourself.
In the “wild”, Marimo only appears in cooler areas like Iceland and northern Japan. It therefore prefers cooler water – if the container/aquarium it’s in gets a bit hot (25+ degrees C/77 F) during summer, consider moving the Marimo to a slightly cooler place for a few months. It can actually be placed in the refrigerator during the hotter months, but a spot near an airco is also fine.
Although Marimo balls can withstand a range of temperatures and water condition, they may turn a strange color – this is an indication that something is wrong.
Marimo turning white / lighter means that it is probably receiving too much light. If the Marimo also seems a bit slimy or if its texture seems otherwise unusual, you may be dealing with a hostile type of algae growing around it. These algae choke the slow-growing Marimo, so it’s best to carefully wash them off or remove them with tweezers.
Marimo turning brown may be a sign that it’s time to gently clean it. If this doesn’t fix it, try carefully picking off the brown (dead) parts and adding a bit of salt to the water to stimulate the Marimo’s growth. If the bottom of the Marimo has turned brown because it didn’t receive light for longer periods of time, be sure to start rolling the Marimo around a bit more often to prevent parts of it from dying off again.
Marimo turning black and/or falling apart Marimo are unfortunately known to start decaying from the inside out sometimes, especially when it has been covered by hostile algae for a while or when it is simply too big for clean water to reach the inside. In order for it to become healthy again, the black parts should be removed and the Marimo should be gently re-rolled. It’ll be smaller than it was before, but it now has a good chance of surviving and growing back just fine.
Recognizing a fake Marimo
Artificial Marimos are unfortunately a real thing – many stores don’t even sell them to you on purpose, but sometimes there is just some confusion. I’ve received a few comments asking about this, so here are some points to check if you’re not sure.
- Artificial moss balls are usually made with a soft plastic ball covered in synthetic hair. The plastic can be quite obvious.
- Most artificial moss balls are not kept in the plant tanks at aquarium stores. They are often sold pre-packaged or in the decorations section. This doesn’t always apply though.
- You can change the shape of a real Marimo by rolling it between your hands. For example, if it’s lopsided you should be able to roll it into a more perfect ball.
- Real Marimos can be picked apart (although I would not recommend this)
- A fake Marimo is often very perfect looking and smooth. If your Marimo has some bumps and looks imperfect, there’s a good chance it’s real.
Buying a Marimo
Although Marimos are not always easy to find in local aquarium stores, they are available online in many places, like The Shrimp Farm or Amazon. Another option is to ask your aquarium store if they can order them for you, or buy them from another aquarist.
If you follow the guidelines from this caresheet, your Marimo can live for years and years and eventually grow quite big or form multiple small Marimos by falling apart. Don’t expect it to grow too quickly, though – Marimos are said to only grow 5mm a year!
I’ve been keeping Marimo balls in my aquariums for quite a while now and they are already easily one of my favourite aquarium plants due to their fun shape and low requirements. I can definitely recommend them for planted bowls and low-stocked aquariums like nano shrimp tanks. Happy Marimo-keeping!