For the past two months I have been having a constant battle keeping my Alk much above 7dkh for any length of time. The problem isn't so much getting the water to the 9 - 10dKH levels I want so badly but rather it is keeping it anywhere near those numbers. As an example this evening I manually dosed enough BRS soda ash to get my tank to 10dKH (tested with Red Sea and Salifert kits). This was at 5:00pm. I retested at 9:00pm and my water was now showing 8.1dKh! So within 4 hours my Alkalinity dropped by nearly 2 points!!! Of course magnesium would be the first culprit although I'm not sure I could even get my tank to 10dKH if my magnesium was low enough that it would precipitate that much out. However my magnesium is always above 1300ppm. When I tested it this evening it was around 1350ppm. My calcium levels stay consistently within the 420ppm range without a problem. No worries there. Tanks stats: 57 Gallon with 20g sump (estimated 55-60g total volume) Light to moderately stocked. No clams, very little coralline algae, mostly LPS - only a couple small SPS Calcium: 420 Mg: 1350 Alk: 7.0 - 7.2 (on average) PH: 8.3 - 8.4 I am dosing around 100ml of BRS two-part each night for a relatively small tank that is lightly stocked and still end up around 7dKH the next morning. That seems like a huge amount. I'm really stumped as to what I am doing wrong or what is absorbing the Alk. I used to be able to maintain all the levels without a problem so not sure what changed. And it isn't caked onto my heaters or pumps. What could cause it to drop so rapidly? Appreciate any input.