a few small film canisters, soaked before hand in water with extra dechlorinator, will do the trick of a DIY snail trap. Punch a few holes in the lid of a canister, large enough for the snails, but small enough so none of your fish can get in. Place some food in the canister, pop on the lid, and sink it in there. Soon they will all be swarming in there, and you can just remove them by the canister.
