Uninvited pond snails often invade aquariums, usually arriving as hitchhikers on live aquarium plants. Once introduced, pest snails multiply rapidly and become difficult to eradicate. Stocking your tank with snail-eating fish helps control their population without the addition of chemicals.
Clown loaches (Chromobotia macracanthus) are the first fish most hobbyists think of for snail eradication. These fish will eat just about any snail they can find. The drawback to these fish are that they get large. In the wild, they can grow more than a foot long. However, in captivity they rarely grow more than 7 inches long. Still, they are schooling fish, so they need to share a tank with other members of their species to feel comfortable. They are also somewhat picky in their tank mates; they'll get stressed by active, fast-swimming fish.
Other, smaller loaches also eat snails. The yo-yo loach (Botia almorhae) and the skunk loach (Yasuhikotakia morleti) both stay under 5 inches in length. They are both schooling fish who tolerate other fish better than they do their larger clown loach cousins. The one caveat in their compatibility is that these species will sometimes nip at long-finned fish.
In addition to the loaches, freshwater pufferfish will eat snails. Puffers even have tiny beaks, well-adapted to crushing snail shells. Several species show up in pet shops. These include the dwarf puffer (Carinotetraodon travancoricus). Of the freshwater puffers, this species works best in community aquariums. They only grow to about half an inch in length, and they prefer to live in schools. Like the smaller loaches, this species will nip at the fins of larger fish. Other freshwater puffers will also eat snails, but most do not get along with other fish.
Fish aren't the only snail-eaters. Assassin snail (Clea helena) are voracious predators of fellow snails. These snails grow to about half an inch long. Adding one to two assassin snails per gallon of aquarium water will rapidly eradicate other snails. After wiping out other snails, assassin snails will subsist on scavenged fish food. As a bonus, assassin snails breed very slowly, so they are unlikely to become pests themselves.