Generally it's not a good idea to get a fish to take care of a problem like snails or algae. When snails are first introduced to a tank, their population is going to explode (this is just simple population dynamics), but they are going to hit a point where resources run out. Then the population will drop to a stable level.
Manual removal of snails is the best way, but you have to posses the stomach to kill them. Put a piece of lettuce in the tank and the snails should be all over it in the morning. Just toss out the lettuce and you take away a large chunk of the population.
Depending on what kind of snails you have, removing them will actually do more harm than good. The trumpet snails that have the really long shells are very beneficial to a tank. They burrow in the substrate and prevent pockets of toxic gas from forming. They will eat algae off the walls, but they only do so at night. I purposefully introduce these guys into a tank because of all their benefits. For pond snails and ramshorn snails, I have found they are great at removing excess algae which is very helpful in an establishing tank.