aragonite/pH/alk relationship

TilleysReef · Mar 2, 2024

One of my tanks I am switching over from a mainly fish/zoa tank to a mixed reef tank. I have used CaribSea Special Grade reef sand in the past, but never worried much about dosing for my zoas. Now, I am a lot more concerned about the parameters and have noticed that I can not raise my pH much above 7.8 without skyrocketing the alk above 12. Since aragnoite buffers pH naturally, if I didn't dose at all my pH would stay around 7.8 and Alk around 9, here are my ideas of options:
1. Forget about the pH since I will always be fighting a losing battle against the pH buffering capacity of the aragonite
2. Just dose to maintain alk at a higher level around 11 or 12
3. Forget dosing altogether
4. Dump the aragonite and get a different sand bottom.

If my pH stays lower, around 7.6 or 7.8 instead of keeping it around 8.2, how much of a detrimental effect will that have on coral growth? Any experience and recommendations would be appreciated.

Randy Holmes-Farley · Mar 2, 2024

Aragonite is not particularly useful to buffer against alk and pH drops since it cannot dissolve unless the pH gets excessively low.

That said, pH 7.6, if accurate, is too low and I’d work to raise it.

TilleysReef · Mar 2, 2024

Randy Holmes-Farley said:
Aragonite is not particularly useful to buffer against alk and pH drops since it cannot dissolve unless the pH gets excessively low.

That said, pH 7.6, if accurate, is too low and I’d work to raise it.

Thank you Randy. I seem to be having an issue getting it to stay much above 7.8 during the day, dropping to 7.6 at night. For example, I tried to increase the pH by dosing only the alk portion of the two part, but even with an Alk of 12.2, my pH still maxes out around 7.85, calcium always stays around 500 without dosing regardless of my pH and Alk levels. magnesium stays around 1300.

This is why I was assuming the aragonite was buffering the pH and causing the low pH issues. I did a test and set a shallow pan of water (increased surface area) outside for an hour, the pH changes less than 0.1, so I don't think carbon dioxide is causing the issue. Aside from dosing I mentioned above, I'm not dosing anything else.

I feel somewhat stuck and don't want to chase pH, but also can't raise it without blowing up my alk, so I'm trying to figure out what to do. Any suggestions would be greatly appreciated

Randy Holmes-Farley · Mar 2, 2024

Aragonite dissolution raises pH, so it’s not causing low pH.

There are a variety of ways to raise pH and not alk, including fresher air to the skimmer, fresher air in the room, a CO2 scrubber, and growing algae in a refugiu
or ATS.