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Hal,

Instead of using chemicals in the pond why don't you just crush them.
Every day go out and crush a number on the side of the pond and the
fish will eat the remains. After the fish get a taste for the snails
they will eat the hatchlings right after they hatch because their
shells are not hard right after hatching. You will still have the
larger snails and if they bother your plants, etc. just keep crushing
them when you see them. If they are not bothering your plants they
will just get big and producing egg masses which the fish will eat as
they hatch.

Tom L.L.