Alkalinity issues

Rodolfo Garcia · 2026-07-07T10:55:36-0400

Any idea why my reef tank has all of a sudden not taken up any alkalinity? I changed up the light schedule by just diminishing sunlight period from 8 to 6 hours. The tank has been at 9.5 to 9.7 alkalinity consistently for the last two weeks and i have no need to dose a supplement. Is this normal? This is making me think the coral are not taking it up and it’s hurting them. My coral were consistently using up 0.3 dkh in 24 hours up until recently and i had been dosing all for reef but i don’t think i need to anymore because the alkalinity will not lower.

Fish Fan · 2026-07-07T11:03:55-0400

What corals do you have? Corals like sofites don't use as much of those elements as their stoney coral cousins.

Rodolfo Garcia · 2026-07-07T11:05:11-0400

Fish Fan said:
What corals do you have? Corals like sofites don't use as much of those elements as their stoney coral cousins.

My coral were consistently using up 0.3 dkh in 24 hours. I have mostly lps and softies.

Fish Fan · 2026-07-07T13:14:02-0400

Short of any changes to the system like lighting or flow, I don't know why corals would suddenly stop uptaking Alk. For that reason, I wonder if this is maybe testing error. If you're using the Hanna Alk Checker, for example, the reagent can go bad rather quickly once opened. Maybe try confirming your results on a different test kit than what you're using now.

#eefsquad any ideas?

@Randy Holmes-Farley do you have any thoughts for the OP please?

exnisstech · 2026-07-07T13:40:41-0400

Maybe switch the light schedule back to where it was and see if they start consuming alk again if it's the only change you made?

Fish Fan · 2026-07-07T15:07:31-0400

exnisstech said:
Maybe switch the light schedule back to where it was and see if they start consuming alk again if it's the only change you made?

That's what I was thinking, and I missed this in the OP's first post. I'm thinking the lesser photoperiod is why they are not using as much Alk.

BeanAnimal · 2026-07-07T15:21:34-0400

The soft coral are not not consumers, and the LPS slow consumers. I would assume that a good portion of your uptake was coralline algae as well. But whatever the case, the shorter photo period is driving less uptake due to lower photosynthesis meaning less direct uptake and also a change in system pH driven by that photo period.

I wouldn't worry unless coral health starts to go south.