How long has the tank been running? What brand salt do you use and do you mix your own salt or buy it already made?
If you are sure the mag test is correct, do you or have you done any dosing of alk
or calcium products? When calcification occurs (hard corals forming their skeletons, coralline algea growing or abiotic precipitation) a defined amount of calcium, alkalinity and magnesuim is used. For every 2 ppm
of mag used, there is aprox 18 ppm
calcium and 2.7 dKH
used. If we assume your mag dropped 250 ppm
(1200 down to 950) then your calcium would have had to drop 2250 ppm
and your alk
. Obvioulsy there isn't that much calcium or alkalinity in seawater, so there is an issue somewhere. It could be a testing issue, bad salt or long term unbalance dosing which would have cause a lot of precipitation.