2 inches!
It sounds to me like overfertilization. Yes, lawn fertilizer can definitely cause this. If this is the culprit, the answer is simple. Restock and don't fertilize your grass so much, particularly early in the spring before the rains.
It's also possible, but not necessarilly likely, that the chlorine in your pool caused a kill. Given the difference in volume between your pool and your pond, I don't know how likely it is. If the pool was superchlorinated, it might have happened.
There are many reasons why you might not see all of the dead fish. Predators (coons, herons) are a likely culprit. Some fish may sink and you'd never see those.
The aerator may also cause fish kills if it's not the correct type or size, or if it's not run correctly. I generally recommend a bottom-draw fountain or bottom diffuser air pump. These should be run 24/7 from March through October in southern climates. Many run them year-round and that's not a bad idea either. There are other variables such as water depth that also have to be considered when sizing an aeration system.
