Low pH with Normal Alk?

[TheEngineer]

I replaced the headunit on my Reef Angel a while back, but I didn't recalibrate my pH probe at that time. My pH was always steady around 8.0 and the levels seemed fairly consistent (albeit the wrong value) since then, so I didn't think too much of it. I finally bought the calibration fluid and calibrated it yesterday. I'm getting a reading of 9.6 now in 10.01 buffer and it is reading 6.6 in 7.01 buffer. So it's reading about 0.4 under what it really is. If I trust that it is consistent across that band, it is telling me the pH of my tank is 6.5 (or 6.9 with the offset).

I did have to start dosing 2-part (I use Two Little Fishies) to get my alk up a few weeks ago. I only dose once a week and only 30 mL of both. My alk is 8.5 and cal is 450. Should my pH be that low? Is the 2-part to blame?

 

[Randy Holmes-Farley]

There's no chance the pH in your tank is 6.5-6.9. Is that really what you meant to say, or do you mean 7.5 - 7.9?

 

[TheEngineer]

There's no chance the pH in your tank is 6.5-6.9. Is that really what you meant to say, or do you mean 7.5 - 7.9?

No typo. 6.5-6.9. So my confusion is well placed

 

[Randy Holmes-Farley]

I think there must be some calibration issue still.

I'd go back again and check the calibration fluids, and be sure it is reading near 10 in the 10 fluid and not, by accident, in the 7 fluid.

 

[TheEngineer]

I think there must be some calibration issue still.

I'd go back again and check the calibration fluids, and be sure it is reading near 10 in the 10 fluid and not, by accident, in the 7 fluid.

I'm redoing the calibration right now. I double checked the sachets Right now it shows 9.54 in the 10.01 fluid. That should snap to 10.01 when I accept the calibration (in theory).

 

[TheEngineer]

OK, it snapped to 10.00 when I accepted the calibration. I'll take that. It's back in the tank and settling in. Seems to be around 7.4-7.5, which is more reasonable, but still seems oddly low to me.

 

[Randy Holmes-Farley]

That is pretty low, but at least believable.

The pH would be low due to excess CO2 in the water, which likely reflects CO2 in the home air. It is not related to the two part at all.

One test you can do is this aeration test to determine the source of the CO2:

The Aeration Test

Some of the possibilities listed above require some effort to diagnose. Problems 3 and 4 are quite common, and here is a way to distinguish them. Remove a cup of tank water and measure the pH. Then aerate it for an hour with an airstone using outside air. The pH should rise if the pH is unusually low for the measured alkalinity, as in Figure 3 (if it does not rise, most likely one of the measurements (pH or alkalinity) is in error). Then repeat the same experiment on a new cup of water using inside air. If the pH rises there too, then the aquarium pH will rise with more aeration because it is only the aquarium that contains excess carbon dioxide. If the pH does not rise inside (or rises very little), then the inside air contains excess CO2, and more aeration with that same air will not solve the low pH problem (although aeration with fresher air should).

 

[TheEngineer]

I thought aeration was an issue too, let me give some more background...

The water from my well has high CO2 (you helped me figure that out a while back). I aerate my RO water for 24 hours before passing it through my DI resin. I then hold it until needed. I suppose this water could still be a source of CO2 into the tank, right?

I've had the windows open recently since the weather has been nice and I turned my skimmer back on yesterday to aerate more. The window is 2 feet from the tank and there were other windows in the house open. That said, I have not done this specific test as you described it. I will give it a shot and see what happens.

 

[TheEngineer]

Oh, I should also add that I'm installing a whole house acid neutralizer this weekend. My incoming pH is 5.5 (professionally tested, not by me )

 

[TheEngineer]

I guess it is my house CO2 level. Aerated water from outside is at 8.1 after 45 minutes.

Gotta go buy some more house plants!

 

[Randy Holmes-Farley]

I thought aeration was an issue too, let me give some more background...

The water from my well has high CO2 (you helped me figure that out a while back). I aerate my RO water for 24 hours before passing it through my DI resin. I then hold it until needed. I suppose this water could still be a source of CO2 into the tank, right?

I've had the windows open recently since the weather has been nice and I turned my skimmer back on yesterday to aerate more. The window is 2 feet from the tank and there were other windows in the house open. That said, I have not done this specific test as you described it. I will give it a shot and see what happens.

If the well water passes through an undepleted DI resin, the CO2 is removed and is not impacting tank pH.

 

[TheEngineer]

If the well water passes through an undepleted DI resin, the CO2 is removed and is not impacting tank pH.

Ah yes. Good point.

 

[TheEngineer]

My daytime pH is now around 8.1, which is great. I see the diurnal pH swings in my tank, which don't in and of themselves concern me, but my pH swings down to under 7 at night. That seems too far to me. I read your article about diurnal pH swings and you mention that so long as the swing doesn't get below 7.5, you aren't concerned. Welllllll, what about when it gets lower than that?

I run my alk around 9, but I suppose I could push it higher. I just did a water change last night of about 20% so I'd expect my alk to be a little higher than that right now and my pH stayed a little higher, but it still dropped to around 6.7. Should I be doing something at night to prevent this? Maybe a reverse lighting cycle with macros in the sump? My system has low nutrients so I'm not sure I'd be very successful with macros.

 

[Randy Holmes-Farley]

I really don't think the pH below 7.0 is real. Live calcium carbonate rock and sand would be dissolving fairly rapidly at pH below 7, just as it does inside of a CaCO3/CO2 reactor, pushing alk way up.

 

[TheEngineer]

I really don't think the pH below 7.0 is real. Live calcium carbonate rock and sand would be dissolving fairly rapidly at pH below 7, just as it does inside of a CaCO3/CO2 reactor, pushing alk way up.

That makes me feel better about my tank's inhabitants, but much worse about my sensor. My alk needs to be supplemented weekly, so I doubt my rock/sand is dissolving. Maybe I need a new pH probe. It's strange that it properly measure calibration fluids though.

 

[Randy Holmes-Farley]

That makes me feel better about my tank's inhabitants, but much worse about my sensor. My alk needs to be supplemented weekly, so I doubt my rock/sand is dissolving. Maybe I need a new pH probe. It's strange that it properly measure calibration fluids though.

Maybe the calibration fluids are off.