They would probably just sit at the bottom of the tank and avoid the current, and the fact you know who Richard Simmons sort of dates you but I knew who he was as well so there you go. But seriously other than separating them I don't see how I am to slim them down if it is obesity, as the other occupants in the tank need to be fed. And when I went away at Christmas my father in law only feed them every second day and at 1/2 daily rations to avoid nitrate build up as he would not do a water change these fish didn't seem to change shape.