How did this snail get in my fish tank?

I bought a snail from the pet store thinking it was cute until about a week ago when it died. I took the shell out and the snail remains and then this morning I found a little black snail in my fish tank. I have no idea where it came from. Is it bad for my fish or will it help?|||while some people like snails in the aquarium , they can quickly spoil things, if you over feed they will reproduce like crazy and take over. trust me..if you wish to get rid of them any commercial product will work,,however if your a do it yourself-er then put a piece of copper into the aquarium, the snails will not last long takes about a week to rid them and it will not harm the fish|||I think it multiplied quickly|||probably a baby from the snail. and eh leave it.|||Usually on the plants you buy at the pet store. The best way to keep from getting these little critters, is to wash the plants before putting them in the tank.|||probably laid eggs, watch out you may end up the tons, but if you dont mind seeing a bunch of snails, dont worry about it they will keep the algae under control. Watch how much you feed overfeeding will cause major overpopulation|||Snails can be transfered to your tank when new plants are introduced to aquarium. Or, depending on the type of snail you had, it could be "Babies!"|||Snails breed really fast. I have purchased plants from my local fish store, now I have snails in my guppie tank. They eat algee. Plus, I can feed them to my Oscars, they are a good source of nutrition for them.|||Lucky you. It's a little snail baby. And look...you got it for free. Keep it.|||The snail you had in there probably had babies.They reproduce like crazy.I wouldn't freak out.They help keep the tank clean