Snails can be very beneficial to a tank. They are good scavengers and will eat small food that the fish won't. I will suggest that you leave them in there. If they reproduce quickly and become an eye sore you can either manually remove as many as possible, or you can pick up some copper based additives at your LPS, which I hear work very well. I hate to add any unnecessary chemicals to my tanks and prefer more natural approaches to extermination, but with the Japanese Shrimp in your tank, anything that would eat the snails would most likely eat those shrimp.