PH and Alk Hypothesis

plippert21 · Feb 5, 2023

Reading other threads and comparing notes, my tank seems to be one of the weird ones that sees a drop of Alk in the last couple hours of the day and really after lights out. This is opposed to most reporting their Alk is consumed throughout the day.

Looking at the Alk and PH trend in the picture below, it appears to suggest that consumption of Alk takes off when the PH is above 8.4 and shuts down below 8.4.

I’m dosing 3L of Kalk consistently throughout the day.

Not necessarily something I’m worried about but this seems like a fairly narrow PH range where Alk is consumed?

Thoughts? Any other interpretations to consider?

Sean Clark · Feb 5, 2023

Your consumption of alk is triggering dosing of alk. Dosing of alk is driving your pH up. I think you are looking at the correlation in reverse.

plippert21 · Feb 5, 2023

Sean Clark said:
Your consumption of alk is triggering dosing of alk. Dosing of alk is driving your pH up. I think you are looking at the correlation in reverse.

Interesting. No doubt dosing Kalk raises PH.

In my interpretation, I am assuming that keeping my Kalk dose constant through the 24 hour period that the rise and fall of PH is related to photosynthesis and the subsequent uptake of CO2.

So holding the dose of Kalk consistent through the 24 period, isn’t the cyclicity seen in Alk pointing to when consumption of it is occurring?

Sean Clark · Feb 5, 2023

You could be seeing the photosynthesis driving your pH up. Your graph resolution is too low to see time of day. Overlay your display lighting watage on the same graph and see what that looks like.

Ideally you could balance your kalk dosing against the photo period and reach a more stable pH.

Add in a refugium and you have more pieces to juggle.

Randy Holmes-Farley · Feb 6, 2023

Higher pH does increase consumption of alkalinity, so it does not surprise me that the demand is highest at the end of the day.

There are three aspects to alk demand.

1. Abiotic precipitation of calcium carbonate. This one will track with pH.

2. Calcification by photosynthetic organisms, such as many corals. This may track with pH, light levels, or both. It is not well understood which effect is larger. I expect it is a combination of both.

3. Calcification by nonphotosynthetic organisms (e.g., snails). I expect this one tracks with pH, but have not ever seen any data.

plippert21 · Feb 6, 2023

Randy Holmes-Farley said:
Higher pH does increase consumption of alkalinity, so it does not surprise me that the demand is highest at the end of the day.

There are three aspects to alk demand.

1. Abiotic precipitation of calcium carbonate. This one will track with pH.

2. Calcification by photosynthetic organisms, such as many corals. This may track with pH, light levels, or both. It is not well understood which effect is larger. I expect it is a combination of both.

3. Calcification by nonphotosynthetic organisms (e.g., snails). I expect this one tracks with pH, but have not ever seen any data.

Thanks Randy. This helps frame it up for me.