a tank is over stocked when the amount of maintenance required to keep up water quality exceeds the amount of work you want to put in. it is really that simple. if you can maintain good water quality, and the fish can move around, it is not over stocked. with that in mind, i caution not to go crazy, as if something goes wrong, the more heavily stocked the faster things become bad. use your judgement on stocking. maintain the water quality, and ask yourself, does the tank look crowded??