FW snails do indeed multiply like bunnies, but most
SW snails either will not reproduce, or if they do, the eggs are usually eaten by the tank inhabitants. Either way, overpopulation of snails in a
SW tank isn't something I've ever seen or heard of.
Anywhere from 10% to 20% should be a fine water change. I don't think there's a need to change out the filter yet, unless it looks particularly nasty. there are no fish, so it shouldn't have filtered much.
Snails do a great job on the film algae on the glass as well.
