How could a 72x18x21 not be enough?I can understand,if I had them all cramped into a 55 gallon or even a 75 gallon fish tank.Majority of the fishes I own,wont even go pass 7 inches mark.The only ones I see going from 8 inches or more is my Jack Dempsey and Tiger Oscar really.Also maybe the Green Terror,but I can't tell if its a female or male yet.