there are several varieties of these, though the naming can get a bit hard on the tongue. I have them in my DT, they are harmless algae/detritis eaters. The problem is, there is ONE that has been known to munch on corals. How to specificall identify it from others im not quite sure. I leave them in my DT so long as i dont find damage to my corals. If i find one on or close to a coral, it goes in the sump. the only other problem i have noticed with them is, if your CUC is low or sparce, these little thing can spread like mad. It splits in half on its own. THough i do remove them when i feel they are behaving in a risky manner, ive never seen any evidence they've harmed my corals.