Calcium and alkalinity are related in a reef, and a balance is very important just like calcium and magnesium have to be at proper ratios to each other for optimum health and absorption. Average NSW values for calcium and alkalinity are 410 ppm and 7 dKH respectively (as per Randy Holmes-Farley).
Balanced calcium and alkalinity levels (as per The Reef Aquarium - Volume III by Julian Sprung and Charles Delbeek):
350 ppm to 1.4 dKH
375 ppm to 2.1 dKH
400 ppm to 5.6 dKH
425 ppm to 9.1 dKH
450 ppm to 12.6 dKH
475 ppm to 16.1 dKH
Magnesium also plays an important role in calcium and alkalinity uptake, and should be kept at 1350-1400.
Even though your levels are a bit off (not by much), I doubt this is the cause of your corals' demise. I would work on getting the NO3 to "zero", and confirm with a second test kit (at the LFS maybe?) that your NO2 is elevated. NO2 should not be measurable in a reef tank. Even though NO2 at that level (and also much, much higher) is not damaging in a saltwater aquarium (unlike freshwater aquariums where NO2 is very toxic) it is a sign that something is amiss in the tank, and also a tell tale sign that NH3 was recently present at a level that exceeds the nitrifying bacteria's capabilities.
How long has the tank been set up? Could you post a full tank photo?
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~ Mindy
SPS fanatic.
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