I have 1 red tiger oscar at 6.5 in, a common red oscar at 7 in, a red bellied pacu that is every bit as big as my oscars, an african clown knife at 11 in, a 4 in green terror, 2 4 in bumble bee chilids ( 1 male, 1 female), another purple and yellow chilid at 4 in, a 6 in midnight cat fish, 3 giant danio sharks, some cory cats, large pleco, and a very large crayfish. They are all in a 75 gallon tank wich I have plans on upgrading to at lest a 150 gallon. I rarely have any problems with fighting, except when the male bumble bee gets in a mood ( that is why he is going back) It's just like others have said, what ever you put in there has to be bigger than their mouth, or faster than they are. Try talking to the person that you buy fish from, they should be able to help you out if they are any good. Good Luck!!!!!!!