In my brackish tank, I use marine salt and measure out my specific gravity using a hydrometer, then match it in water changes. My preferred brand is Instant Ocean. It is readily accessible for me and not too expensive when bought in a largeish bag.
But breeding nerite snails is no easy task. Nerites have a larvae stage, in which they are incredibly difficult to feed. This, I believe, is where most people fail. Some articles say that the eggs will hatch in fresh water, but the larvae do not survive. Others argue that they only hatch in brackish and must be transferred between brackish and marine (I think) and fed a particular diet of spirulina, natural algae, and phytoplankton. These new hatched nerites are so small you cannot have any kind of filter on the tank.
Only one website I have found gives actual step by step instructions on how to successfully raise the nerite larvae.