Then your vernal hardness (GH) if it is 1 degree or higher you have enough calcium. shrimp cannot eat cuttlebone or egg shells because it is essentially rock. But these will slowly dissolve in a tank if the water gets slightly acidic. And once it is dissolved the shrimp can use it. However keep in mind that there is a lot of calcium and magnesium in vegetable or plant material that shrimp fee one. So if they are getting enough food they should have enough calcium and magnesium. Shrimp need calcium and magneisum and if you don't have 1 degree of GH they might not be getting enough through the water and could have difficulty molting.
People also recommend cuttlebone for calcium due to snail shel erosion. However like shrimp if the snail is getting enough algae or other food it has enough calcium. For snails the real problem is acids in the water. Acids will etch away the snail shell over time. Putting a cuttlebone which is mainly solid calico carbonate will react and neutralize the acids in the keeping the PH very close to 7. Helping to prevent snail erosion.
