I had the EXACT SAME PROBLEM! Petsmart talked me into taking 2 TINY snails. BIG BIG MISTAKE! They completey took over my 29 gallon tank. I threw them away every day... 2, 3 times a day I swiped them off the edges of the tank. I emptied the tank, cleaned it, got rid of ALL OF THEM... I thought, before I set it back up. NOPE. Must have been something in the pump... rim... gravel... somewhere. Infestation AGAIN! I had to let the tank go. For weeks. I covered everything in salt, and them boiled all the gravel, threw away the plants. Weeks later I set it back up, no problems and NEVER again will I accept another snail. Sorry I wish I could help you but I looked everywhere, and no one had a solution to this problem.