The calcium is supposed to help prevent staining and to prevent cobalt leaching from the gelcoat. Cobalt staining is very common in fiberglass pools and looks like little black dots that seem to grow like a crystal. I keep mine at about 250 ppm. I used to keep it lower but I had iron stains more often. Since I increased the hardness of the water I have had much less iron staining in the pool.Didn't even need to use any ascorbic acid this year!