Calcium and magnesium in the water enter the body of a shrimp via three main sources: A) consumption of food , B) via absorption directly from the water, mainly through the gills and digestive tract but also through other soft tissues during osmoregulation, and C) direct absorption from the exoskeleton to be shed immediately prior to molting. The second pathway is extremely important during molting, as it provides a supplemental source to calcium being used to harden the exoskeleton at a time when the shrimp cannot eat. This absorption of minerals through soft tissues is also why shrimp are so vulnerable to poisoning via metals such as selenium, lead, copper, zinc, mercury, cadmium, etc. Osmoregulation will result in the concentration of a particular mineral in tissue reaching an equilibrium with the concentration of that mineral in the water the shrimp is living in. In the case of selenium, for example, once the selenium reaches a certain concentration it interferes with the production of DNA and proteins (both functional proteins such as enzymes and structural proteins such as tendons and muscles) by substituting itself for sulfur in the synthesis of the DNA and said proteins. The same effect happens in fish as well, but invertebrates tend to be affected at much lower concentrations that lead to a rapid death, rather than being tolerant of and subject to a higher, long-term accumulation that may cause deformities in offspring or problems with healing in vertebrates.
So, like dougall said, changing up and providing a high calcium food is probably the easiest and best way to supplement the calcium intake for the shrimp, especially since maintaining constantly adjusted water parameters can be difficult. I never thought of it cuz I've never had to deal with the problem. But as I was thinking about it, I realized there was a very specific reason why hardness was important, due to osmoregulation and crustacean physiology. So don't underestimate the issues soft water can give you. If you can raise the hardness and maintain it easily, I think it would go a long way to helping your shrimps' survival rate during molting.
WYite
