Most snails reproduce in direct proportion to the food supply. If the tank has little uneaten food, dying plants or algae, and organic waste, snail populations are small. When there's lots of food, many more eggs are laid and the hatchlings survive. For some species, however, population control is simple--apple snails lay eggs above the water line in a large, foamy-looking blob, easily removed or prevented with a high water level.
Pretty well all my tanks have snails--some very few because of predators in the tank, others a moderate population that helps keep the tank clean. If either population start increasing, it means I need to up my maintenance--not that the snails are somehow bad.
