Snails do NOT overwhelm properly managed tanks. They reproduce in relation to the amount of food in the system, so if you control the nutrients (ie, feed lightly but appropriately for the fish, clean the system regularly), the snails will never get out of hand. Snails are a valuable part of my tanks--they help clean, provide a food source for some fish, and don't cause any problems. Adding chemicals is a bad idea, since they all come with some side effects, and adding any fish just inceases the bioload of the system.