We use "mosquito dunkers" in our landscape ponds. The active ingredient Bacillus thuringiensis subspecies israelensis (Bti), and is highly specific to mosquitos.
The issue with mosquito fish for us is they relish frog eggs, and we maintain the ponds for the "spring peepers".
My neighbor has an enormous koi pond, with enormous koi. My bees love that pond over all others --- he salts the water heavily as it a treatment for fungal diseases in the expensive and delicate oversize goldfish.
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