High ph issue in soft coral tank

[Shamama Tul Amber]

Good morning reefer,
I am a researcher working on soft corals like bebryce echinogorgia anella and menella sp particularly.
I had setup two 125L aquarium tanks, filled with natural seawater(perform water exchange after every two weeks 20-30%), small white gravel stone as substrate, one powerhead filter, one sponge filter each, with ten hours of LED lightening.
This system has been running smooth for the last three months. However, since the February started my pH keeps crossing the limited mark like at first it was 8.6 - 8.7 and gradually rising. Now its 9.3 - 9.4 or more in each tank.
I have tried making the room more airy and increasing the aeration. Adding chemicals like Vinegar, HCl. (I had removed the gravel from one tank other than these two. But there is no change in pH). Only hcl had worked for 2-3 days and now the pH keeps on increasing. At this pace I'll lose all my corals.
What do I do?
(Attaching reference pictures of my tank)

 

[Mebbid]

I would start by purchasing a co2 monitor off of amazon, they are reasonably priced and can tell you what the ambient co2 level is which could contribute.

What are your other parameters? Are you testing much other than pH?

Are you purchasing or collecting the natural seawater?

Are you adding anything else to the tank?

 

[Randy Holmes-Farley]

No need for a meter. Indoor pH is never low.

I expect that pH is not accurate. Magnesium hydroxide would be precipitating all over if it were, lowering pH and alk).

pH is never too high in an aerated reef tank unless you dose hydroxide. If it is high here, more aeration will lower it.

I would not be adding acids.

I expect pH measurement is the issue, but do this aeration test and if it stays high then it is test error.

pH and the Reef Aquarium - REEFEDITION

by Randy Holmes-Farley For many aquarists, pH is not something that they have much experience with aside from their aquarium. For many, pH is almost a black box measurement: something to be considered, but whose physical meaning makes little sense to them. This article will Read more here...

www.reefedition.com

The Aeration Test

Some of the possible causes of low pH listed above require an 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 its pH. Then aerate it for an hour with an airstone using outside air. Its pH should rise if it is unusually low for the measured alkalinity (Figure 2). Then repeat the same experiment on a new cup of water using inside air. If its pH also rises, then the aquarium’s pH will rise simply with more aeration because it is only the aquarium that contains excess carbon dioxide. If the pH does not rise in the cup (or rises very little) when aerating with indoor air, then that air likely contains excess CO2, and more aeration with that same air will not solve the low pH problem (although aeration with fresher air should). Be careful implementing this test if the outside aeration test results in a large temperature change (more than 5°C or 10°F), because such changes alone impact pH measurements.

 

[Shamama Tul Amber]

Hey Mebbid.
I am taking other parameters like salinity, temperature and dissolved oxygen thrice daily. Salinity is around 33-35 ppt, temperature is 27-29 degrees, and dissolved oxygen is around 9.0-9.9 mg/l. For the seawater, I am collecting it directly from the sea. And I am just adding feed to my tank, i.e., Polyp Lab Reef roids.
In early December 2025, pH was dropped to 7.7, so then i did add Ca(OH)2.

 

[Shamama Tul Amber]

No need for a meter. Indoor pH is never low.

I expect that pH is not accurate. Magnesium hydroxide would be precipitating all over if it were, lowering pH and alk).

pH is never too high in an aerated reef tank unless you dose hydroxide. If it is high here, more aeration will lower it.

I would not be adding acids.

I expect pH measurement is the issue, but do this aeration test and if it stays high then it is test error.

pH and the Reef Aquarium - REEFEDITION

by Randy Holmes-Farley For many aquarists, pH is not something that they have much experience with aside from their aquarium. For many, pH is almost a black box measurement: something to be considered, but whose physical meaning makes little sense to them. This article will Read more here...

www.reefedition.com

The Aeration Test

Some of the possible causes of low pH listed above require an 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 its pH. Then aerate it for an hour with an airstone using outside air. Its pH should rise if it is unusually low for the measured alkalinity (Figure 2). Then repeat the same experiment on a new cup of water using inside air. If its pH also rises, then the aquarium’s pH will rise simply with more aeration because it is only the aquarium that contains excess carbon dioxide. If the pH does not rise in the cup (or rises very little) when aerating with indoor air, then that air likely contains excess CO2, and more aeration with that same air will not solve the low pH problem (although aeration with fresher air should). Be careful implementing this test if the outside aeration test results in a large temperature change (more than 5°C or 10°F), because such changes alone impact pH measurements.

Hey Randy,
Thank you for sharing such a detailed method. I am going to perform this right away and report back to you.
The thing is earlier when this pH issue was surfacing, I did suspect my pH meter first, so to be sure, what I did was to take another calibrated pH meter and test the water of my tanks. But the pH remains the same. That's when I started to worry.
Back in December 2025, pH dropped to 7.7 in both tanks. I used Ca(OH)₂ to increase it.

 

[Randy Holmes-Farley]

Hey Randy,
Thank you for sharing such a detailed method. I am going to perform this right away and report back to you.
The thing is earlier when this pH issue was surfacing, I did suspect my pH meter first, so to be sure, what I did was to take another calibrated pH meter and test the water of my tanks. But the pH remains the same. That's when I started to worry.
Back in December 2025, pH dropped to 7.7 in both tanks. I used Ca(OH)₂ to increase it.

Adding hydroxide can get the pH into the 8.4-8.7 range the same day it is added. Months later, it has no impact except by the total alkalinity. If the alkalinity is not super high (like 50 dKH), the pH is not accurate. :)

 

[Shamama Tul Amber]

Adding hydroxide can get the pH into the 8.4-8.7 range the same day it is added. Months later, it has no impact except by the total alkalinity. If the alkalinity is not super high (like 50 dKH), the pH is not accurate. :)

So, you're saying that I have to check alkalinity to get to the root cause. Isn't it?

 

[Randy Holmes-Farley]

So, you're saying that I have to check alkalinity to get to the root cause. Isn't it?

Well, sort of, but you should be measuring alkalinity even if you measure nothing else in a reef tank.

There's no scenario where pH over 9 is stable in a reef tank. But if the alkalinity is super high from overdosing alkalinity supplements, then one could get a pH in the high 8's.

I don't think chasing obscure possibilities is the best bet when pH measurement issue is the most likely, followed by inadequate aeration.

 

[Gnop1950]

Well, sort of, but you should be measuring alkalinity even if you measure nothing else in a reef tank.

There's no scenario where pH over 9 is stable in a reef tank. But if the alkalinity is super high from overdosing alkalinity supplements, then one could get a pH in the high 8's.

I don't think chasing obscure possibilities is the best bet when pH measurement issue is the most likely, followed by inadequate aeration.

Just checking since I'm getting ready to add a CO2 scrubber to my system to see if I can raise my pH a bit in my new tank.

But his problem was that the PH is too high, wouldn'tmaking the room more airy decrease the proportion of CO2 in the room, making less CO2 and more oxygen available, then adding additional aeration exacerbate the issue, or am I missing something?

 

[Randy Holmes-Farley]

Just checking since I'm getting ready to add a CO2 scrubber to my system to see if I can raise my pH a bit in my new tank.

But his problem was that the PH is too high, wouldn'tmaking the room more airy decrease the proportion of CO2 in the room, making less CO2 and more oxygen available, then adding additional aeration exacerbate the issue, or am I missing something?

I was talking about aerating the water, not the room.

Aeration can raise or lower pH, but aside from heavily scrubbed air, there is no scenario where aeration can raise the pH if the pH is above 8.4 because no air has less CO2 than normal outside air.

Sure, aeration with higher CO2 air could drop pH further or faster.

In any case, a pH in the 9’s with any reasonable amount of aeration is test error.

Excessively high pH is just not a real thing in any reef tank with reasonable aeration.

 

[Gnop1950]

I was talking about aerating the water, not the room.

Aeration can raise or lower pH, but aside from heavily scrubbed air, there is no scenario where aeration can raise the pH if the pH is above 8.4 because no air has less CO2 than normal outside air.

Sure, aeration with higher CO2 air could drop pH further or faster.

In any case, a pH in the 9’s with any reasonable amount of aeration is test error.

Excessively high pH is just not a real thing in any reef tank with reasonable aeration.

Ok, I was wondering. My current PH is 8.06, so adding a CO2 scrubber to my skimmer, would it be reasonable that the PH would tick up a bit? I live in southern Arizona, and daytime temperatures will soon regularly exceed 90-100°F. Between that and the dust, opening the windows in the fish room is really not an option.

 

[Randy Holmes-Farley]

Yes, a scrubber will likely boost your pH. The extent will depend on how good the scrubber is at removing CO2, and how much the skimmer aerates vs the tank top (which fights against the scrubber).