A few snails in the tank help clean the algae off the glass and eat rotting debris in the gravel. However, a few snails can become a few hundred snails quickly. A few hundred snails will eat any and all live plants and create a toxic mess. Ridding the tank of an overpopulation of snails takes perseverance and a combination of aquarium snail controls.
Pick out any snails you see by hand. While not the most effective technique it will help reduce the population.
Add fish to the tank that will eat the snails. Cichlids, loaches, gourami and cherry barbs will eat snails.
Remove any plants that you can and inspect them for snail eggs. Run the plants under water and brush off the snail eggs.
Remove the snail's food source. Thoroughly vacuum the debris in the gravel during water changes and only feed your fish as much as they will eat in one feeding. If food is falling to the ground you are probably feeding too much.
Bait the snails. Attach a leaf of lettuce with a clip to the side of the tank. Within a few hours you should have a collection of snails on the leaf that you can now throw out. A slice of cucumber or zucchini works well for this too.
Chemical products marketed to kill snails in a tank can also be harmful to the fish in the tank.