Ooooh, now you're in for it! Snail little ones will be plentiful this summer! Sprats, or some such sticky name.

BT is apparently good for a lot of pests, because it has also been recommended to help in the garden. You don't want to release it around beneficial larvae, but it's good for knocking out various critters that bore into squash plants and such.

My ex-brother-in-law had a hot tub with an ozone bubbler (or something comparable) that cleaned the water without chlorine. I wonder if one of those might also be beneficial in a pond?

SRS