Another option is to take a small soda bottle, cut the top "funnel" part off, put a rock or other weight in the bottom with some lettuce, put the top back into the bottle pointy end first and place in the bottom of the tank.
The snals should find thier way into the bottle and not be smart enough to find thier way out. Keep an eye on the "trap" if you have any small fish and release them if they get trapped.
Side note the above can also be used to catch small hard to catch fish just bait with pellets or other foods.