I have 15 black Japanese trapdoor snails, they're great, as they are live bearers and not hermaphroditic. So the females only pop out one little fully formed snail at a time. I started with 5 and now many months later only have the 15. They are constantly scouring the gravel and glass for algae...
I've got them in a 20 gallon tank. I have 8 Rasboras, 1 algae eater and like 15 Black Japanese Trapdoor snails in the tank. The crabs ranged in size from dime size to maybe nickel size. All were alive and promptly hid under rocky or woody items in the tank. Just checked on them around 9 pm EST...
I'd have to say it depends on the type of snail. Some eat plants, others algea. Some are hermaphroditic and lay hundreds of eggs, and can easily overcrowd your tank. Others are live bearers and only lay a single live snail every so often. I personally have Black Japanese Trapdoor Snails, which...
I just purchased 5 from http://www.bobstropicalplants.com/shop/en/ @ $4 a crab plus shipping.
Should be arriving via USPS tomorrow, I'll post their condition and whether I would recommend the seller.