Besides the appearance of 1000s of snails. Nothing much. I've find them in my canister filters, but no damage done as far as I can see. They borrow through the sand and if you have black sand, they can definitely be a distraction from the appearance of the tank. I have a Botia to keep it under control, but it doesn't make a dent in the population. I also don't have any plants in that tank anymore.