I think the elements of a fish kill are there. Very high miomass in fish...very large catfish. Many tadpoles...not enough fish to eat them. Many of dead fish wont float, or not for long. The coons will clean up in a hurry. Very high biomass with a sudden loss of aeration/circulation is what I am looking at. What time of year did the pump stop? There should be some fish left, but with all the structure they are scattered. You wont do much good with a cast net. Get a couple of $6 minnow traps, from Walley world. They are cylindrical steel mesh, with the ends like cones pointing inward. They come in 2 halves with a giant safety pin holding the halves together. Bait it with fish food or bread. As Ewest said, the HBG will not perpetuate with numbers of LMB to forage on them. I am surprised you got this long out of them and is encouraging to folks who have them and with all the negatives that have been posted in regards to what may happen after a few generations. It was probably a slow decline exacerbated by the loss of aeration.