@viridisornatus @NoGi I'm pretty sure the shrimps we keep have the bulk of their exoskeletons made up of calcium carbonates. Although some structures in crustaceans are made up of a calcium phosphate and carbonate mix. The possible use of the two forms of calcium on different parts of their body is for hardness and elasticity. For example, their mandibles need to be harder, so you'd find more phosphates there. While carbonates are more elastic for easier moulting.
Shrimps, like fish, can absorb calcium from the water. If calcium is abundant enough in the water, the shrimps will obtain the majority of their calcium through their gills. Calcium in diet makes up the remaining source.
It doesn't matter if the calcium is in the form of sulphate, chloride, nitrate or phosphate. The key is a balance with Magnesium. Magnesium keeps the calcium in a dissolved state. Magnesium also converts Vitamin D into its active form so that it can help calcium absorption.