If excess debris builds up on the surface, cleaning a Marimo Moss Ball is easy. Just take it out of the tank and gently swish and squeeze it in a bucket of aquarium water. Think of it like a sponge. Squeezing it in aquarium water forces the water out and releasing it draws the water back in. If its getting a little out of round, cup it and gently roll it around. If it floats after being put back in the tank its because water is out of the middle. It will fill up and sink to the bottom again.