I had two Indos in a 10 gal quarantine tank and one started to pick on the other so they went into seperate tanks. Right now I have 2, 4" AT in the same tank and they get along fine. The two Indos went into a 135 with 23 other baby Indos with lots of hiding places and I do not see any more aggression, except when two fish try for the same food. They range in size from 1" to about 2.5". I hope to add the AT to the tank when the Indos get large enough.