Snails are part and parcel of a fishtank I'm afraid! They tend to arrive with plants that you've introduced to the tank. Are you possibly overfeeding your fish? As this could be helping them to reproduce with alarming regularity.
There are products you can buy to help eradicate 'em, but personally I'm reluctant to introduce anything chemical to my tank, so I resort to doing it the old-fashioned way- scooping them out and throwing them outside.
In one of my tanks I have female fighter fish and they seem to eat them! I find snail shells littering the tank floor on many occasions, so the problem is solved in that tank at least. I wouldn't however introduce these type of fish to a normal community tank, they'll terrorise every other fish in there.