Different source water can have different issues. While it's been said you can throw tums in the tank (it will break down completely, and has calcium), it will make the water a little cloudy. What I do is make some of the food I put in the tank. I'll take a 2 size baby food (sweet potatoes, squash, or carrots so far) and throw it in a blender with some egg shells and other vegetable matter, usually the debris from making omelets, and then throw them into the freezer. My wife found this awesome ice cube tray that makes itty bitty ice cubes. It'll take all day (pour, freeze, remove into a baggy, repeat until the stuff is gone), and then I'll throw one or two of the cubes in daily. RCS and fish attack it, and the snails eventually run over either a cube or the egg shell particles. I have an orange/black spiked nerite that has had marked improvement in shell quality since I started doing that.
Yes, nerite eggs are quite unsightly and nothing eats them. /sigh. Oh well. For what it's worth, I have a tiger nerite, the orange/black spiked (i thought it was another tiger until it got big enough for spikes to appear), a normal yellow/black spiked, and a zebra.
