You say you could smell the decaying fish, 24 hours after applying the product? Any chance some of these morts occurred a day or two before the product was added?
And Jason, not everyone has a well emptying into their ponds. We don't, and haven't suffered a fishkill in over 40 years of accumulated fish wastes. And we run a heavy biomass on top of that.
Ponds are not aquariums.