i put a smallish snail in my tank the other week and after a couple of hours, i couldnt see it. i thought it would turn up but it hasnt. is there a chance my fish might have eaten it accidently or something?

Some snails, including pest snails, can survive out of water for awhile so it may have crawled out. More than likely though its hiding, or your fish ate it. Anything small enough to fit in their mouths may, so thats a good rule to go by. Snails shouldnt be harmful for your goldfish to eat provided it wasnt too big.