excess CO2 and PH drop

[collins]

I have been experiencing a constant PH drop in spite of other otherwise normal chemistry. According to the article http://reefkeeping.com/issues/2004-09/rhf/index.htm, the alkalinity that I have, 180 ppm (3.6 meq/L or 10.08 dKh), should yield almost a PH of 8.4 at normal CO2 levels.

My PH falls below 8.2 during the day, unless I tinker. Based on the article, excess CO2 is the culprit. I have been using Seachem’s “Reef advantage calcium� which contains Calcium Chloride to keep my levels up, but am considering switching to kalkwasser (Calcium Hydroxide) because from what I have read it will bind free Carbon Dioxide present in the tank water and convert it to Bicarbonate ions Ca++ + 2(OH-) + 2(CO2) <==> Ca++ + 2(HCO3-). I am at a loss as to why my CO2 levels would be so high, even during the day.

I have tested the effect of moving my PH meter (calibration reconfirmed a few days ago) and a water sample near an open window and as expected the PH level does rise.

Would this proposed change to kalk really give me the results I am looking for or does the CaCl2 in the Seachem product already react with CO2 in a similar way?

For those who will ask, here are other water parameters:
NH3 = 0 ppm
NO2 = 0.1 ppm
NO3 = 5 ppm
PO4 = 0 ppm
Ca = 440 ppm
SG = 1.025
Temp = 79F

Thanks in advance for the replies.

[Agu]

How does your tank look ? "Depressed" pH used to drive me nuts until I realized I was chasing numbers while the tank looked great. Finally just sold my pH meter...........

Not a chemist but I've read that long term use of most calcium supplements other than Kalk will throw off other aspects of water chemistry.

[collins]

My tank looks fine Agu. It is lightly stocked and all the wee beasties seem normal. I am just trying to keep it that way. My PH has been very stable until recently and that is why I am concerned.

[Agu]

My tank looks fine Agu. It is lightly stocked and all the wee beasties seem normal.

Well that's what counts :) .

You may want to check some of Randy's articles in the Chemistry forum but your description sounds right on. It doesn't happen right away but over time some calcium supplements can change water chemistry.

[collins]

Thaks again for the reply Agu.

It looks like I will have to make the switch. I was just hoping to avoid the whole drip dose nightmare.