[unclejed]

Magnesium is one of the major minerals found in natural seawater and all sea salt mixes. Maintaining a magnesium concentration between 1300-1400 ppm is essential in a reef aquarium, as it will ensure optimal coral growth by facilitating the maintenance of other necessary parameters at natural seawater levels. However, magnesium is rapidly depleted from the aquarium through a variety of biological processes — it is used up by the organisms in the system. When the magnesium concentration falls below 1300 ppm, problems may arise in maintaining adequate calcium levels (420-430 ppm) and carbonate alkalinity (7 dKH). Some experts will go as high as 1500 on the high end, A more reasonable range will be 1280 to 1380. So, you are low, however I don't think that should have thrown the calc off. Bring your mag up as it benefits all your corals and other denizens of the deep. I still believe it was the salt change. The new one might have something in it that buffered the calc and caused it to hold solution better. This may sound good, but if the calc is not readily available to be used and "holds" then the calc goes up and your corals can't use it adequately.