i've got a tiger oscar and an albino oscar in my 75 gallon. both are about 4" now. they're in with a albino channel cat which is around 5". the channel cat will eventually end up in a pond, but the oscars are fine. my water changes are 25% every saturday. no problems with nitrites or ammonia. you just have to take everyones opinion here with a grain of salt. most will throw out opinions without ever having kept said fish. they're simply repeating what they've heard.