Got me an Alkalinity question!

[ScubaSteve]

For the past while (I guess since about August when I really started watching it closely) I have been noticing my alkalinity dropping pretty quick. Historically I have found my tank to do best around dKH 9 and I prefer to keep it around there. I have found that my calcium stays pretty much constant between 420 and 460 ppm (changed a bit when I changed my salt mix) but my alkalinity will drop from 9 to 7 in about two days and will drop to about dKH 4 or 5 pretty quick after that if I don't do anything about it. So this leaves me adding carbonate fairly regularly.

What I am wondering is: is my alk dropping quick because it is being used up by my all my SPS or is there something else happening in my tank that is causing it to drop so quick?

I have suspicions that alk swings may have triggered an STN event this summer so I an trying to nip this and make it more stable.

My tank sits near a window and, up until the past week or so when it hit -9C, the window is open and gets decent air flow and it well aerated. The pH until the past few weeks has been on the lower side (7.8 to 8.1) but now sits around 8.2 to 8.4 (day/night).

Params:

Ammonia: 0 ppm
Nitrite: 0 ppm
Nitrate: <5 ppm
Phosphate: 0 ppm
Calc: 440 ppm
Alk: Wacky

Appreciate your input!