Snails,

They are deadly, once you get them you will have them for life. I used the rid a snail, followed directions and killed all my guppies with that stuff. If using it be careful. The LFS here do not recommend that stuff at all.
Best way, a clown loach will definatley eat them but will not get rid of them forever. They get into the filters and lay eggs and they keep a nice supply of food for Mr. Loach.
I have also done this, not recommende unless you know exaclty what you are doing.
I took a whole tank apart, rinsed the grave, filter, and tank with a bleach solution. Threw out the media from the filter and replaced it with all new stuff. After the bleach thing that sat in the in tank, gravel and filter for 5 days, lots of floating dead snails. I then rinsed till I could not rinse no more. Very very important you rinse repeatedly. I then set up the tank again, with no live plants as you will almost be guaranteed of live snails using them. I had no snails at all. But you then have to cycle all over again.
So my recommendation is, don't worry about the snails to much. They can be unsightly but will not harm anything really. My 25 gal has hundreds of them, they don't bother me to much but I am going to get a clown loach to put in there for some cleanup on these little buggars.
Good luck and have fun....bugsy
