I have experienced this too a much lesser degree on prior builds. I know that there are several biological processes that can cause this to occur, having said that, I have never experienced it like this, and I can't correct it. I set this tank up like I have done all of my tanks, dry rock and "live sand", and biospira + ammonia. The tank finished cycling on Day 10. In those 10 days, the tank had dropped 1.3dkH to 7.3 dkH. Pretty normal in my opinion. I started ghost feeding the tank a small amount of pellets or mysis so I could maintain the bacteria population without adding ammonia (so I could add a few corals). I have been ghost feeding daily ever since. Here are my some notes and test results related to Alk/Calcium and any other significant changes. Day 12: Dosed brs 2 part equivalent to 1 dkH/7ppm calcium to try and reach 8.3 dkH and 420ppm Calcium 8 hours later, 8.4 dkH and Calcium 420ppm. over 1 hour Day 18: Alkalinity: 7.1 dkH (Hanna), 7.3 dkH (salifert) This is a 1.3 dkH drop in 6 days. Calcium: 410ppm (Red Sea) Magnesium: 1425ppm (salifert) Dosed BRS 2 part equivalent to target 8.3dkH/420ppm Calcium over 3 hours Day 19: Added co2 scrubber (ph was between 7.9-8.1, raised ph to 8.2-8.37) Added 3 torch frags Day 20: Alkalinity: 7.2 dkH (Hanna), 7.3 dkH (Salifert) This is a 1.1 dkH drop in 48 hrs Dosed BRS 2 part to target 8.4 dkH/420ppm Calcium (assume calcium is balanced still) over 3 hours Day 21: Alkalinity: 7.0 dkH (Hanna), 7.0 dkH (Salifert) This is a 1.4 dkH drop in 24 hrs Calcium: 400ppm (red sea) Dosed BRS 2 part to target 8.3dkH/ 410ppm calcium, this time spread over 12 hours 3 hours later- Alkalinity 7.3 dkH (Hanna), 7.5 dkH (Salifert) This is when I pretty much confirmed I was having significant precipitation issues by dosing too much or too fast. OR the soda ASH in brs 2 part was raising my pH too high and causing precipitation (Since my ph now stays between 8.2-8.37). I say pretty much because there are no signs of significant precipitation in my tank, no cloudy water, no crusty pumps or heaters, there is some precipitation on the bottom of my sump but does not look as significant as the amount of dkH I am losing. Regardless, I thought since i was ending up right back where i started i would just try a very slow addition of .05 dkH via kalkwasser drip (only .02 pH increase) overnight to gradually raise the dkH, and i would remove my co2 scrubber if it started getting too high. That was last night, and this morning my Alk tested 6.5 dkH (Hanna) , 6.3 dkh (Salifert) Neither test kit/reagents are expired and I even confirmed results with a brand new Red Sea Alkalinity PRO. Thoughts? My theory is that this is precipitation that is just hidden somewhat and I simply need to lower my pH or use sodium bicarb instead of soda ash. Or, there is still a biological process occuring here that can be that powerful? Could there be something wrong with my saltmix or water? Diatoms cant consume alkalinity right? I am pretty confused. Thanks!