I have a 2000 gallon back yard pond with several plants and a few Tilapia. I have an external filter and also an aeration fountain. I decided to fertilize the pond with 15-30-15 granular fertilizer from Home Depot for the plants and produce more algae for the fish to eat. Before fertilizing the water quality was near perfect. I put about one cup of the fertilizer in a cloth sack and dropped it in the water for 4 hours then took it out with about half a cup remaining. After fertilizing the water quality reading went way too high for nitrite, ammonia and phosphate. Many experts suggest fertilizer but in general, wouldn't that increase the three bad chemicals way too high? Confused. Has anyone else had this problem? Now I'm doing a water change to get things within range again.