It all depends on your levels, and overall happiness of the fish IMO. If you have what would be known as an overstocked tank, none of the fish you have swim in the middle or top, just all at the bottom, you may have overstocked the bottom area, but it would not nessecitate being "overstocked". Vary your fish, try to get moderate amounts of each. Less is better, you'll know if you have too many.