Are you saying that visibility has been regularly 18-20 inches before fertilizing?

Just wondering--why did you fertilize? I'm certainly no expert on pond chemistry, but around here 18-20 inches of visibility is considered ideal for producing good numbers of fish. Has your pond been known for having any suspended inorganics such as silt? If not, then it seems like your visibility readings were pretty indicative of reasonably fertile water before you fertilized.

Holding a redear sunfish is like running with scissors.