You have to treat everything in order to remove snails. They are in the filter too. Unfortunately if you like to kill snails you have to kill your beneficial bacteria too.
My method is 10 min dip in 10-15ml hydrogen peroxide to 1gal of water.
You can use noplanaria. It will kill all the snails i thing .It should keep bacteria alive. Ask here for more info.
Another proven option is Flu
bendazole dosed to the water column.
Not to be confused with Fenbendazole. Flu won't harm shrimp of fish and is 100% plant safe. Know what you handle but 10% Flu dosed correctly kills a number nasty's along with all snails I have ever exposed to it.