Is a calcium blend ( with cacl2, nacl, kcl, mgcl2) such as many brands sold online okay to use for increasing calcium hardness?

How do you know if low calcium ( 130 ppm in concrete pool; water source (well) is the same. ) is being precipitated out and causing the sandy sediment on the bottom of the pool that Polaris 480 cannot pick up versus dead algae? Sand/dirt in the area is black not white (ah, maybe it is being bleached!!). I think the calcium has been at that level for 2 years. Just ordered a sand/silt bag for the Polaris.

Maybe the sand in filter should be changed to pick up these particles!?! Pool installed in 2005.

Are there reasonably good sources in the south for quality products?

Are there any other chem effects I need to be aware of when I add the calcium? (kaboom?) I do not want to create more milky appearance either.

As always, your wisdom is greatly appreciated!