That isn't my tank. It is in the fish lab at the zoo. Those are an Oreochromis species that may be extinct in the wild. It is part of a species maintenance program the zoo is involved in. My first thought was the tank was seriously overcrowded, but as you can see in the photo, the tank is immaculate, as are the rest of the tanks in the facility. I guess my point is that stocked to the limit is not easy to define, and some tanks such as Malawi tanks, are best stocked to the limit to maintain peace. Of course, doing that means more maintenance, but that is part of the deal. My ideas of what a tank can hold were radically altered when guppy master Stan Shubel spoke at our CAOAC convention a few years ago, and related how he uses 10 gallon tanks for growout and puts 80 males in there. These tanks used a simple box filter and had one weekly water change of around 50%. Depending on which of his strains they were, the value of those fish could be $4,000.00. Clearly, you don't raise quality fish by overcrowding them.
Clearly the Oreochromis are crowded, but the tank is not overstocked, in the sense that the fish are healthy, and the water quality is high, due to the strict maintenance regimen.