i have 5 in my 55g tank.
i got them when they were about the size of a nickel, now they are about 3-4 inches and are doing fine. I throw in a few nightcrawlers a day, and they eat that and leave each other alone. when they get about 6 inches, i will get a bigger tank. right now they have much swimming room, all i have in there is a few plants and one cave and some driftwood. They seem to love it