I know what you mean however, overstocking can cause them not to thrive as they could if they had ample room and great water conditions. Its better for them to thrive alone then be subjected to conditions that can cause them stress even if its a little. We all know how hard it is to resist buying more fish. Think of it like people in your house, if you have three bedrooms in your house, your parents have one room and the other two may have a bed or two in there for the kids that live there already. Now think if all the sudden you had 3 more people move in. Sure there is room in the rest of the house but how comfortable is everyone going to be.