Nerite snails prefer mostly algae. You must make sure you have abundant supply of algae otherwise, they will starve badly from lack thereof. They are very difficult to acclimate on other supplemental foods.
How big is the tank?
What are your KH and GH?
You need to increase your pH as that is not enough to stop the shells from pitting. Nerite snails consume a large amount of calcium that makes them very prone to shell pitting once the calcium supply is used up.