I use the smaller tankz for grow out of the younger fish. I've had my 20gal full of 18 baby Geophagus sp. 'Red Head Tapajos',4 Satanoperca jurupari, 3 baby Crenicichla sp. 'xingu II', and 4 Cory cats..can't member the species. The largest fish were the pikes at 3"...haha...largest geo was 2". I had the tank WAY over filtered (two filters rated for 60gal tanks each..haha) to make sure things stayed in check and did water changes weekly. As they grew I gradually would move them up to the next size tank and so one. Key thing is watch your bio-load. Over do it or not have enough filtration and you have problems. The fish got along nicely and it actually wasn't that crowded....not something I would recommend as every day practice though.