High pH, low Alk and Ca

[Makara23]

I went on a 3 week vacation and had premixed Kalk at 1 teaspoon/gallon for a family member to fill the ATO.

Before I left, tank parameters were normal:
dKh: 8.3
Ca: 420
pH: 8.0 (night), 8.2 (day)

After I came back, it was:
dKh: 7.5
Ca: 380
pH: 8.35 (night), 8.6 (day)

Now I understand while we were gone, there was very little CO2 in the house to neutralize the pH. But it is normal that pH is abnormally very high, while calcium and alk is very low? I want to add in more kalkwasser by saturating the ATO to bring up the calcium and alkalinity, but I'm afraid this will raise the pH even higher to a dangerous level.

How do I remedy low Ca and Alk while not further raising pH? Add kalk but also vinegar?

 

[Miami Reef]

I’m skeptical. Recalibrate the probe. Kalkwasser itself at 7.5 dKH will almost never cause pH to rise like that unless if there was absolutely no aeration whatsoever.

 

[Randy Holmes-Farley Verified Community Expert]

7.5 dKH is not very low. It may be lower than normal for your tank, but it is higher than many tanks and higher than the ocean. I recommend 7-11 dKH.

It is expected the alk might decline if pH rose because that makes demand rise.

pH 8.6 at peak is ok, and I agree it may not be accurate unless aeration is low, or you are using a lot of the kalk. . If it is accurate, more aeration will lower it.

I personally would not worry about it.

 

[Makara23]

Thank you both! I will recalibrate the Apex pH probe (haven't done so in 1.5 years).

So if I wanted to slowly increase calcium, the best course of action is to increase Kalk dosing concentration and not mind the pH? I can also dose only Ca, but I would think the balanced approach of Ca/alk from Kalk is probably better?

I'll try to dig up an air pump and air stone bubbler, otherwise not sure how else to increase aeration (no skimmer for this system).

 

[Randy Holmes-Farley Verified Community Expert]

Thank you both! I will recalibrate the Apex pH probe (haven't done so in 1.5 years).

So if I wanted to slowly increase calcium, the best course of action is to increase Kalk dosing concentration and not mind the pH? I can also dose only Ca, but I would think the balanced approach of Ca/alk from Kalk is probably better?

I'll try to dig up an air pump and air stone bubbler, otherwise not sure how else to increase aeration (no skimmer for this system).

No. If you try to boost calcium with kalk, alk will rise by 2.8 dKH for each 20 ppm calcium rise.

Use calcium chloride when you want just calcium.

 

[mcarroll]

there was very little CO2 in the house

Just wanted to agree that I think this (household CO2 level) is what changed. A decrease in CO2 would move your tank's equilibrium point higher, all else being equal. Would be excellent if you had a CO2 air quality monitor to quantify that.

I also agree I wouldn't worry about it. :)