I've kept single 4" gills in a 20 gallon and it did just fine for 6 months until I found a 55 gallon free. I keep a 5" gill, 3" gill, 4" crappie, 3" pumpkin seed, and a crawfish in my 55 gallon but they are soon moving to my 175 gallon thats in the process of cycling right now. They have done just fine in the 55 gallon but they have ALOT of places to hide from each other.