I would like to add a bit of information to this thread as I have only had an amphibian for a short time I have been keeping marimos for years.
For starters Marimo are algae not moss. No if's and's or buts, they are a species of algae from Japan. The ball is actually thousands and thousands of individual algae growing together in one form
They are native to a few lakes in Japan where they receive only minor amounts of light.
Personally I keep them In all of my tanks because they look nice and provide lots of surface area for beneficial bacteria growth.
You should squeeze them out lightly in the water when you change your tank to loose the gunk inside but you should never unroll the Marimo and if it becomes unrolled you should roll it back up and softly squeeze it together probably putting in a tank by itself so your pets don't rip it up.
I have read several stories of black worms burrowing into marimos and killing them, I think that with cleaning the marimos often this shouldn't be a problem but if you use live blackworms to feed then marimos may not be the best choice for you.
As a note, Marimo are very cheap but they are slow growing and need very little in terms of care. Avoid direct sunlight keep light levels low or at least give them dark from time to time, if your tank gets really hot during the summer it's best to take the Marimo out and store it in your fridge.
There are 2 other types of "Moss balls" that pet stores will sell. The first kind is harmless and the same as any plastic aquarium plant.
They come in a plastic package and while ive never had one they should be harmless
The other type of Moss ball is dangerous and you should not buy them ever!
These are not marimos. This is Moss balled up and tied together with fishing line. The Moss can and often will rot in your aquarium water and if they come apart which they often do your pet can easily swallow the fishing line and choke, or cause a significant amount of internal damage.
In closing I'm all for Marimo, Jabba loves to push his around and curl around them
But I am against 'Moss balls' and fake marimo
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