Alkalinity going down way faster than calcium or mg?

[bamf25]

I have a 120 g/dt tank that is about 10 months old. I have a nice collection of 20 or so small to mid size corals mainly soft and lps. There are currently 5 fish in the tank. Ammonia is 0, nitrite is 0, nitrate is around 5, Calcium is 455, mg is 1350, PH is 8.0, but the dkh had fallen to about 6.4. I do roughly 15% water changes every 10 days or so. I generally use kalkwater, but had been going lighter with it because my calcium was getting up near 500, and wanted to get that down some.

Overal,l the corals look good, but I have 1 plate coral and one chalice that have had some lose of flesh. I am actually worried about the plate coral making it. Otherwise, the frogspawn, hammer, acans, flavia, gsp, mushrooms, and zoas look great. I started corrcting the akl this morining.

Any idea why I would get such a high use of alk while my other values have been very stable?

[disc1]

Check out this article:

http://reefkeeping.com/issues/2004-12/rhf/index.php

From it:
One of the most common complaints of new aquarists is that their aquaria seem to need more alkalinity than their balanced additive system, such as limewater, is supplying. While there are reasons this may actually be the case over the long term (these will be detailed later in this article), frequently these aquarists are seeing a "chemical mirage" rather than a real excess demand for alkalinity.

One of the interesting features of seawater is that it contains a lot more calcium than alkalinity. By this I mean that if all of the calcium in seawater (420 ppm; 10.5 meq/L) were to be precipitated as calcium carbonate, it would consume 21 meq/L of alkalinity (nearly 10 times as much as is present in natural seawater). In a less drastic scenario, let's say that calcium carbonate is formed from aquarium water starting with an alkalinity of 3 meq/L that it is allowed to drop to 2 meq/L (a 33% drop). How much has the calcium declined? It is a surprise to many people to learn that the calcium would drop by only 20 ppm (5%). Consequently, many aquarists observe that their calcium levels are relatively stable (within their ability to reproducibly test it), but alkalinity can vary up and down substantially. This is exactly what would be expected, given that the aquarium already has such a large reservoir of calcium.

So for a 1dkH drop in alkalinity, you should only expect to see about 2 or 3 ppm drop in calcium. That means you might have to use up 10dkH worth of alkalinity before you change calcium enough to notice with the test.

Also if your salt mix has really high calcium, then your calcium is going to go up to near the level in the salt mix with regular water changes. If demand is low and the calcium level in the salt is too high, then you may need to dose alkalinity alone without any calcium until demand is higher.

[bamf25]

Thanks