They need some calcium for their shells. Look for "Snello" recipes online for DIY ideas. Some people powder eggshells (bake them first) to put in the tank, but I have found that to be too messy, and it's hard to get the shells into a powder. You want a powder so that the snails don't cut themselves on sharp edges. It is easier to drop a piece of unflavored cuttlebone (sold in the bird section) into the tank for them, but you may have to weigh it down with a rock if it wants to float. I also give my snails algae wafers or vegetables. The mystery snail likes zucchini, squash, carrot, red cabbage, green beans (canned, no salt, organic if you can find it). Any fresh vegetables will need to be boiled 15-20 minutes because they need to be very soft for the snail to eat.
This is the only time I ever saw the nerite eat anything besides algae or glass slime (he is eating zucchini).