Carful with them as they can become a very big pest and take over quickly. I always remove them when I see them. I believe they reproduce by splitting and also releasing spores. The can release spores when stressed so carful if you do remove. I general boil water in a teapot, fill a syringe and then blast the mouth of them. Takes 2 or 3 times but it gets rid of them. I have seen tanks over taken by them and not so good..