I have always found dead fish and expired frogs floated to the surface,
probably from the internal gas of decomposition. Herons may not be your
only problem. We have herons, cranes, snapper turtles, water snakes and
bullfrogs who were cleaning out our ponds. In desperation to have fish, we
finally gave in and netted them both.

I feel like I've asked you before, but did you try a dog?

C//