Ca/Alk confusion

[DenverDave]

Hi,

I am confused a bit frustrated over my Ca/Alk balance. I have a fairly new 75 gallon tank (2.5 months) that has had coral for a month. At the beginning of this month, by Ca was at 400 ppm at Alk was 9-10 dKH (Salifert). Throughout the month, I have had light demand (little coraline growth, 3 small frags of SPS, 2 smallish LPS, and some soft corals). Using BRS 2-part, I have measured ALK weekly and adjusted accordingly, but have also added and equal amount of the Ca.

Now, the confusing part. I tested Ca for the 1st time this morning, and it read 360 ppm. When I boosted my tank to 400 ppm, it took over 10 OZ, which is 3 of those BRS measuring cups- for my situation, this was a lot of calcium!

My question is, what in the world would cause Ca to be 3 cupfuls low over a month's span when I have been adding equal parts of a 2-part recipie?

Possibilities:

Instant Ocean is out of whack. Well, I just tested the make-up water at 35 ppm at Ca came back at 425 ppm. This was a "transition" batch, which contained 6 cups of salt from the bottom of an old bucket and then 1 cup of salt from the top of a new bucket- but still, this doesn't seem like the source.

BRS Calcium solution is somehow dilluted. Well, I added a specified amount to the aquarium to raise the Ca by a predicted amount (according to the online calculators), and it raised it (according to the test) by EXACTLY that amount, so I don't think this is the issue.

Salifert Test is bad: Well, I'm pretty sure the paragraph above verifies the kit's accuracy.

Magnesium is low. But, then, wouldn't I have trouble maintaining Ca and ALK? In other words, if I have been adding equal amounts, then how do I explain 3 cups worth of imbalance over a month?

Any help would be appreciated.

BTW, Nitrates are undetectable (salifert), macroalgae is growing and healthy, I know to within a gallon the volume of my tank, etc.

Dave