My first recommendation, get a new test kit...one that actually gives you dKh, meq/l or
ppm results...actual physical numbers. I'd suggest SeaChem or Salifert, although when I've tested my Aquarium Pharmaceuticals brand kit against them, I get the same results, and it's a lot cheaper.
You probably have low alkalinity, and if you've got poor gas exchange, the
CO2 in the water will lower your pH as it creates carbonic acid.
You're probably not dosing enough reef buffer either. I suffered from the same ailment until I was given this webpage:
http://home.comcast.net/~jdieck1/chem_calc3.html
It has lots of common buffers and calcium supplement brands...you input current levels, and desired levels, along with tank volume, and it gives you the exact amount to give that increase to the desired level. Of course you can't add too much all at once, but in my case, I was doubling the dose that the label gave for calcium (1/4tsp) every 2 days. Turns out I needed 7.6 full teaspoons to get to my goal level. So I simply up'd dosage to daily (which I admit I should have been doing all along) and used slightly rounded teaspoons. Now my calcium is where it should be, and is easy to maintain with the smaller doses per the label.
But of course you'll need a different test kit so you know what your alkalinity/carbonate hardness is at. You'll want about 3meq/l or 9-10 dKh or 170ppm.
