What sort of tank is it? Clown loach are the ultimate snail assassins.

Snails
Snails in and of themselves aren't harmful to an aquarium, and many aquarists purchase and care for snails in their tanks. These mollusks eat uneaten fish food, dying plants, and algae, and they can be interesting tank inhabitants. The downside of snails in an aquarium is that they increase the bioload (the burden placed on your bio­logical filter), and an out-of-control snail population can be unhealthy for your fish and any other organisms in the tank.

Though some hobbyists add apple or mystery snails to their tanks deliberately, the common pond snail or rams­horn snail can show up on its own and quickly blanket an aquarium with offspring. Snails or snail eggs can enter an aquarium with plants or new fish that are added to the tank. They multiply quickly and can soon number in the hundreds (or even thousands, in very large aquariums).

The solution to a snail problem depends on what you want for your aquarium, whether you only want to control your snail population or whether you want to eliminate them completely from your tank.

How to Control the Snail Population in Your Aquarium
Cutting back on how much you feed your fish will slow the increase of a snail population, but it won't reduce their existing numbers. To reduce the number of snails in your tank, take the following steps:
1.Put a piece of lettuce or a small dish with a few pellets of fish food on the gravel in your tank when you turn out the lights. The following morning, the remaining lettuce or other food (that your fish doesn't consume) should be covered with snails. Simply remove the lettuce or dish from the tank.
2.Use a net to sweep any snails off the aquarium glass and remove them.
3.To drastically reduce the number of snails in your tank, add a few fish that eat snails, such as yo-yo or clown loaches. Keep these fish in schools of at least three to six, and they will make quick work of the snails, sucking them right out of their shells. If you don't want to keep the loaches permanently, leave them for at least a few weeks after you no longer see snails so that any snail eggs in the aquarium can hatch out.
** info from another site indicates that the snail eaters will do a much better job if they are hungry so underfeed slightly when trying to reduce overpopulation so the yo-yo and loaches will be motiviated to eat the snails **

How to Eliminate Snails from Your Aquarium
The only safe way to eliminate snails from your aquarium completely is to tear down the tank and soak the filter, the gravel, and all decorations in a concentrated salt solution or in a dilute bleach solution. You’ll also have to dispose of any living plants in the tank. Even one remaining snail egg mass on a single plant is enough to create a whole new population of snails.

If you want to keep your tank snail free in the future, chemically treat any live plants to kill snail eggs before adding the plants to the aquarium.

freshwater tank

Coldwater or tropical? Clown loaches are tropical, there are plenty of coldwater loaches but I don't know which ones are effective snail-eaters.