The path to hell is paved with good intentions: https://en.wikipedia.org/wiki/Cane_toads_in_Australia

It might well be safe, but the question is what are the downstream affects of reducing the population of mosquitos (or other bugs in the water that this substance impacts). Maybe they've exclusively positive, but I assure you determining that is no walk in the park.

The mosquitoes don't exist in isolation though. Isn't this exactly how food chains / ecosystems work? You can't remove a tier without consequences.