Controlling snails in the aquarium can be a tall order. Chemicals designed for snail control tend to sicken fish and kill plants. Using fish to eat snails is a better option for snail control, but you must take care in your selection of snail-eating fish. Many snail-eating fish have compatibility issues. Some are aggressive and will fight other fish. Additionally, most fish that eat snails will also readily consume desirable invertebrates, too.
Many loaches have a reputation for eradicating snails in freshwater aquariums. The clown loach is probably the best known for this ability. However, clown loaches may grow larger than 8 inches, making them too large for many tanks. However, smaller relatives, like skunk loaches and dojo loaches, will readily eat smaller snails.
Both freshwater and saltwater puffer fish have beaks designed to crush snail shells. However, many freshwater species of puffer fish can be antisocial, and some require brackish water to really thrive. Dwarf freshwater puffer fish do well in pure freshwater and usually leave other fish alone. However, dwarf puffers will nip at long-finned species of fish and are small enough to get bullied by larger fish. In saltwater tanks, the only concern with snail-eating puffers is that they may attack desirable invertebrates, as well. Most puffer fish will eradicate snails quickly.
The cichlid family is not known for snail-eating, but many species will eat snails sometimes. African cichlids are probably the most likely to eat snails. However, this is not of much use, since African cichlid tanks rarely have plants due to the high water pH. On top of this, cichlids are not the best snail-eaters; they may eat the occasional snail but won't actively seek out snails like many algae-eating fish will.
Marine blennies have a reputation for eating algae in marine aquariums. However, a few species may also consume smaller snails species. Blennies will probably not rid an aquarium of snails but may pick off the occasional nuisance snail.
Wrasses make great snail exterminators. However, these fish can be temperamental and may not get along well with other aquarium fish. Additionally, they may go after more desirable invertebrates. However, one species of wrass, the eight-lined wrass (Pseudocheilinus octotaenia), will actually eat parasitic snails off Trinidad clams without harming other inverts. Many species of wrass exist. Always research the particular species before adding some to your tank to make sure they won't cause trouble.
Triggerfish are snail-eaters who often declare open season on other invertebrates. Additionally, triggerfish can get really pugnacious and fight other aquarium fish. Like puffer fish, they have specialized jaws that can crush hard-shelled invertebrates. Always research your particular species before adding it to the tank; some triggerfish are less aggressive than others.