Congrats on the snails, I'm sure you know what they eat (pretty much anything ) but greens such as spinach and cucumber and fish food is also a good diet for them (if in the fish tank they obviously won't reach they fish food in time unless you over feed which is not really a good idea!
Also a bit of calcium in the water works a treat for them as they grow (obviously their shell increases dramatically in size) a way I did this was cuttlefish bone (bought from petstore and sold for birds (their beak As far as I know) ) also you can get calcium tablets from health stores I think