Is this a snail or a parasite in my fishtank?

yesterday i got new fish from the petstore. i think there might be fin rot in the tank or some sort of disease but im not sure. so i just put these new fish in the tank yesterday and i just dumped them in with all the water from a bag.... today i noticed that there is something sucking on the gl of the fish tank. it looks as though it has a shell and it has 2 like antennas or something.... i was wondering if this is a snail or if there is any possibility this can be a parasite.... its about the size of a bebe bullet or a small pebble.... is there any possibility its a parasite or something bad.... by the way it sucks on the gl and on the rocks