One of the most important items to consider is Dissolved Oxygen and Tank Loading in your fish tank. You overload a tank’s dissolved oxygen supply and you will experience a fish die off back to the number of fish that can be supported by the dissolved oxygen in that tank.
In commercial operations, you had better have a lake or pond aeration system for each tank that keeps the dissolved oxygen at 6 to 12 ppm or better (How did he learn that?) Those silly little air stones and cute little pumps simply do not hack it.
A couple times a day I check dissolved oxygen with one of these meters: http://www.ebay.com/itm/Milwaukee-MW600-Dissolved-Oxygen-DO-Meter-S...
See photo attached.