Fact: it is only possible to dissolve so many solids into a given volume of water (calcium, carbonates, and everything else). At the risk of oversimplifying the dynamic, imagine a bowl that holds one hundred marbles representing the total dissolved solids in seawater in a given system. If red marbles represented calcium, and blue marbles represented carbonates (alkalinity), the bowl can still only hold one hundred marbles no matter what mix of color they are. Now, if seventy marbles were the equivalent of 400-ppm calcium and the remaining marbles were blue, the only way to increase calcium would be to displace alkalinity (to remove blue marbles). In troubled systems, the misapplication of calcium supplements (dosing suddenly or to excess) is known to cause a sudden precipitation of carbonates (the alkalinity falls/crashes) that is commonly referred to as a “snowstorm”. It is instigated by the influx of a large or rapid amount of calcium entering the system that spikes the pH immediately surrounding carbonate molecules and causes a crystalline precipitation (fallout). In keeping with our analogy, a “snowstorm” would be like taking another bowl of one hundred red marbles (calcium) and trying to pour it into the original bowl of mixed, colored marbles (balanced calcium and alkalinity). The result is the displacement of all blue marbles (carbonates/alkalinity) and the overflow of excess red marbles beyond the one hundred marble limit. The ramifications of this in an aquarium is a crash in water chemistry and water quality that cannot be corrected while the chemical reaction occurs. Dosing more supplements to try to correct the imbalance (or even doing a concurrent water change with hopes of dilution) will only serve to feed the chain reaction. Tragically, the “snowstorm” must be allowed to finish and an aquarium system is traumatized in the process.
so its better that you quit that until your Ca reading is between 400-420.