When we used to have tropical fish (something I really miss), we used to have something called a Pleco which is related to Catfish I think.  I bought it cause I was told it'll help to keep the tank clean, and by that, yes, it ate shit of the bed of the tank.  You could take it out of the tank and hold it in your hand, felt like sandpaper!

Crap, I hope Moriador was not suggesting I'm a shiteater
At first I thought it was something to do with aphrodisiacs!