Originally Posted by KevinC
Yes - nitrate sponge! Problem is you can't get rid of it later. The smallest leaf will reproduce, so you can't really count on hand-removal if you decide later you don't like it.
I agree that it is nearly impossible to get rid of it later. I started out with 4 and within a month there was barely any light getting to the rest of my plants. I tried to skim it from off the surface with a net, but just a few very tiny leaves brought it all back.