What kind of snails? If they are pond snails or trumpet snails, it is highly likely you will have LOTS of snails very soon! If you want to prevent this, you should drastically cut back on feeding the tank. Make sure all food is eaten at the surface by the fish before any gets down to the bottom of the tank. A few snails in the tank is usually pretty beneficial as they keep the tank pretty clean. I enjoy my trumpet snail populations very much.