i keep a black arowana, soon a super red arowana, angel fish (altum), inspector plecos, queen arabesque plecos, and discus i find the discus keep to themselves and same with the angels, you wont have any problems with an aro in that tank but i suggest getting an asian on as the SA ones grow HUGE... or at least bigger than the asian ones. other things i've seen are dats, clown loaches (pretty cool actually they would school right above the ray as it cruised around). otherwise just make sure its not a very aggressive fish and also make sure its not a very small fish, Mine took out some cardinals (on purpose!) cool to watch