1. Overstocking. For every gallon of water in a tank with a filter, you can keep 1 inch of slim-bodied fish or ½ inch of fat-bodied or deep-bodied fish.
2. Overfeeding. Feed fish what they can eat in 2 minutes, maximum.
3. Overcleaning. Change 10 percent of the water once a week, or 25 percent once a month.