Sorry for the late response thank you to everyone who responded. It is about a 30 acre pond/lake on private property. We are cooling machines with the water and putting the "waste" water back into the pond.

Rotenone is a great idea but there are some visitors and the smell of dead fish is a concern. Is there possibly a way to get the fish to sink?

I believe it is in fact a mosquito fish. Stocking a non breeding fish would probably be the most immediate solution.

Any other alternatives you might have?

Thanks again