Why does dKh fall in saltwater holding tank

[watchguy123]

I have a 60 gallon chemtainer saltwater holding tank. I use ro/di water mixed (spectrapure UHE with carbon and chloramine and sediment filter changed every three months---0 TDS) with Marin tropic salt and mix to 1.026 salinity and the alkalinity is always very close to 8 dKh. But a week or so in the chemtainer and the dKh falls to just above 6. I don't understand how the alkalinity can drop 2 full point in a sealed chemtainer holding tank over a week or two.

 

[Randy Holmes-Farley]

There is probably either some precipitation of calcium carbonate, or settling of calcium carbonate solids that were still suspended during the initial testing but not in the second test (and which dissolve in the test and get counted).

Raw salt water can be more likely to have this precipitation than is tank water or seawater due to having less organic matter and less phosphate, both of which can inhibit precipitation of calcium carbonate. The very high local concentration on first mixing also encourages precipitation.