Low PH, normal alkalinity


New member
I am having an issue with my ph being too low. It is at 7.65. However, my alkalinity is at nine. What could possibly be the cause of this. Going to do a water change this evening, but I feel as if that won't be enough.

Other parameters:

I am also running GFO and carbon, not sure if it makes a difference.

Thanks in advance for the help.


Team RC
The first thing I would question is the reliability of the measurement -- don't wanna go making changes without an accurate measurement. How are you determining your pH? A recently calibrated pH meter or a test kit?

Assuming the measurement is accurate, most of the time, a low pH means high CO2 concentration in the air around the tank (or from a calcium reactor). Opening a window to air out the room I think would be the first order of business if possible, along with increasing surface aeration :) After that reassess. (as a note, do you have a calcium reactor?)
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Premium Member
If the measurement is accurate, the problem is carbon dioxide. More aeration might help. Does the tank have a skimmer? I'd also check that the water surface doesn't have a film on it, and has a lot of visible agitation.

You could aerate a cup of water outside for 3 hours. The pH should be much higher. If it doesn't rise, then I suspect the pH kit or the alkalinity kit has failed.


New member
Hmmm....well, no I am not using a calcium reactor. There USED to be a film on top before I put on two MP40s about two weeks ago. Have not done a water change in last two weeks. Doing one tonight.

There is no surface film now, and I am running a skimmer, SWC 160 in the sump.

As far as testing goes, I used API drop test, and it was hard to tell really, but around 7.8 or 8. I just bought a new ph probe for reefkeeper 2, and used 7 and 10 to calibrate. That is where reading is coming from. I then REcalibrated the next day to ensure accuracy, and that is where I am at.

On a side note, is there a correlation between mg levels and ph? I just learned that my mg level is very low, 900. Alkalinity at 9, and calcium at 420. Soooo confused.


New member
I'd be real suspicious of your Mg test since both your Ca and Alk are in line. I'd re-test and/or get a second opinion on that test. You could start to use some kalk in your top off water, which will utilize some of the CO2, thus helping to bring your pH back up. But, ultimately, I think once the weather moderates, and you can open some window's you'll see your pH improve.


New member
I just purchased the Salifert MG test kit. Retested three times. My intention was to start dosing, for ca and alkalinity, but bought the mg additive from BRS just in case I needed to. I was under impression that it really didn't move as much as this.


Premium Member
I agree that the magnesium kit might be off, but magnesium has no direct effect on pH, so that's no the source of the problem. Assuming that the alkalinity measurement is correct, the pH problem is caused by carbon dioxide, or somehow the measurement is bad.

I might try measuring the pH of a cup of water, and then aerate the cup of water outside for three hours, and check the pH. If it doesn't rise, the measurement is wrong. Otherwise, the air inside the house has a lot of carbon dioxide in it.