Epsom salt (magnesium sulfate) 1lb will last a long time foremost people, unless you have a very big tank. If you do by it make sure it is pure with with no sent orcoloring added.
in a 10 gallon aquarium 1.92 grams (equivalent to about 1/4tspp +1/8 tsp) will provide 5ppm (milligrams per liter) and boost GH by about 1 degree.
I used
Rotalabutterfly (a fertilizer calculator for aquariums) to calculate the above for a target dose of 5ppm.
You can uses Gypsum (calcium sulfate) along with Epsom salt are the primary ingredients in sulfate GH boosters like seachem Equilibrium.
Other GH boosters are made from calcium chloride and magnesium chloride (chloride GH boosters)
Chloride and sulfate GH boosters work very well for boosteng GH. And typically plants like 3 parts calcium tone part magnesium (a 3 to 1 ratio). Plants and fish need calcium to live.
The most important thing you can do for your snails to insure your pH is as close as possible to 7 or more. If the PH drops below 7 the nail shell is start to dissolve. What I have done is I put a sea shell in my aquarium where the water flow around it is high. if the PH goes slightly acidic the sea shell will dissolve and and push the PH back up to 7. This helps prevent errosion of the snail shell. Since I am also using RO water with a GH os zero I make a GH booster of calcium chloride and magnesium sulfate.