Help Looking for opinions on measuring CO₂ via pH drop

[nivliw]

I'd like to hear other people's thoughts on using pH drop to estimate CO₂ levels.


I'm in a rather strange situation. First of all, I use two good-quality digital pH meters, both properly calibrated and well maintained. I've been using this type of equipment for years because, before getting into aquariums, I was (and still am) heavily involved in hydroponics.


Anyway, here's the situation:


My fully degassed pH (water left degassing indoors for 72 hours) is 7.6. I know that degassing indoors theoretically results in a higher equilibrium CO₂ concentration than the commonly referenced 0.6 ppm achieved outdoors, but let's use that as a reference point.


With CO₂ running, the aquarium reaches a pH of 5.6, giving me a pH drop of 2.0.


Using the standard ph drop relationship and assuming 0.6 ppm CO₂ at equilibrium, that would suggest roughly 60 ppm CO₂.


However, both on this forum and in various Facebook groups, I often see people reporting pH drops of around 1.0–1.4. Some have even claimed to measure about 60 ppm CO₂ with a Hanna meter while having only a 1.4 pH drop, using water degassed outdoors.


If that were true, then extrapolating from those numbers would suggest that my tank is running at something like 240 ppm CO₂, which seems highly unlikely. My fish appear perfectly fine, and my drop checkers aren't even yellow. In fact, I have three different drop checkers in the tank, using different indicator solutions and placed in different locations, just out of curiosity.


So I'm wondering:

• Do you think the pH drop method is being misinterpreted by many hobbyists?
• Could there be something fundamentally wrong with the assumptions behind comparing pH drops between different tanks?
• Has anyone here measured dissolved CO₂ directly and compared it against pH drop calculations?

I'd really be interested to hear your thoughts on this rather confusing situation.

 

[Scaper26]

After experimenting with the hanna test kit vs my pH meter, I have come to realize that if you extract water from your tank and degas it overnight indoors (I have degassed outdoors and gotten an extra 0.2 increase in pH), you have to hit a pH drop of 1.5 in order to hit 30 ppm of CO₂ with reference to your degassed sample (1.7 pH drop if you've degassed outdoors). So if you want to hit 60 ppm, you would have to go 1.7-1.8 pH drop from your indoor degassed sample or about 2.0 pH drop from your outdoor degassed sample.
I think most people don't actually degas their sample so meticulously; neither do they have good pH pens to record pH (most use API liquid tests). So for them, if they have a partially degassed sample from their tank (tank water in the morning before CO₂ injection) and they drop their pH about 1 point, it puts them pretty close to 30 ppm. Just my two cents on the topic.

 

[JayP]

After experimenting with the hanna test kit vs my pH meter, I have come to realize that if you extract water from your tank and degas it overnight indoors (I have degassed outdoors and gotten an extra 0.2 increase in pH), you have to hit a pH drop of 1.5 in order to hit 30 ppm of CO₂ with reference to your degassed sample (1.7 pH drop if you've degassed outdoors). So if you want to hit 60 ppm, you would have to go 1.7-1.8 pH drop from your indoor degassed sample or about 2.0 pH drop from your outdoor degassed sample.
I think most people don't actually degas their sample so meticulously; neither do they have good pH pens to record pH (most use API liquid tests). So for them, if they have a partially degassed sample from their tank (tank water in the morning before CO₂ injection) and they drop their pH about 1 point, it puts them pretty close to 30 ppm. Just my two cents on the topic.

There are a number of other posts here regarding this, but I believe this is dependent on your kh, specifically if kh is below 1. @Naturescapes_Rocco can explain it better than I, though.

 

[nivliw]

There are a number of other posts here regarding this, but I believe this is dependent on your kh, specifically if kh is below 1. @Naturescapes_Rocco can explain it better than I, though.

From what I've always understood, KH shouldn't affect the magnitude of the pH drop itself. In theory, if you start at the same dissolved CO₂ concentration (for example 0.6 ppm) and increase it to 60 ppm, the pH should drop by roughly the same amount regardless of KH.

KH would affect the absolute pH values, but not the relationship between the starting and ending CO₂ concentrations when looking at the pH drop alone.

That said, this is exactly why I'm opening this discussion. There seems to be a lot of conflicting information on the topic, especially when KH gets very low, so I'd genuinely like to understand the chemistry behind it better if there's something I'm missing.

 

[Scaper26]

There are a number of other posts here regarding this, but I believe this is dependent on your kh, specifically if kh is below 1. @Naturescapes_Rocco can explain it better than I, though.

I did read some of that content. I think one of the points made was that phosphates affect pH and hence pH drop. I don’t know how accurate that is, as we’re looking at the delta pH and not the absolute value of pH itself. I’m thinking of conducting some experiments myself this summer, when I’m able to free some tanks up. But for now, I subscribe to Dennis’s outlook which suggests that KH doesn’t significantly affect pH drop unless you’re in the very high range.

 

[Koan]

Here is @Naturescapes_Rocco 's take

Resource  Article 'Dialing in CO2'

Quick note on pH drop as a tool for CO2: I've tested extensively in different KH tanks, and find that "pH drop" methods vary as wildly as trying to read a "ppm number" from drop checkers colors. A 1.0pH drop (from degassed) does not equal 30ppm in low KH tanks, or high KH tanks. The range seems...

• Naturescapes_Rocco
• Dec 3, 2025
• Updated Mar 10, 2026
• 4 min read
• Reaction score: 2
• Category: CO2 in the Planted Aquarium

 

[Scaper26]

Here is @Naturescapes_Rocco 's take

Resource  Article 'Dialing in CO2'

Quick note on pH drop as a tool for CO2: I've tested extensively in different KH tanks, and find that "pH drop" methods vary as wildly as trying to read a "ppm number" from drop checkers colors. A 1.0pH drop (from degassed) does not equal 30ppm in low KH tanks, or high KH tanks. The range seems...

• Naturescapes_Rocco
• Dec 3, 2025
• Updated Mar 10, 2026
• 4 min read
• Reaction score: 2
• Category: CO2 in the Planted Aquarium

I did in fact read his article and I do agree with him on starting CO2 as early as 5 hours before lights on. But I don’t think his article presents the data for CO2 ppm for given pH drops for tanks with high and low KH. I would love to see the data so that I could try and replicate it this summer.

 

[Naturescapes_Rocco]

The gist is this: the way pH probes work means that at super low KH, other acids and molecules can affect readings. For example, PO4 concentration can affect pH readings when KH is 0-1.

pH methods for determining CO2 (like hanna titration, or pH drop methods) will usually overestimate CO2 amounts when the KH is 0-1 because of this.

Not too big of a deal; I had no problem aiming for 50ppm CO2 with the hanna kit when running a zero KH aquasoil system. Was it 50ppm CO2? No. Did the test kit measure 50? yes! It worked just fine, didn't gas my fish, had plenty of CO2 available for my tank's needs.

That's part of why people underestimate drop checkers -- they function independently of KH because of the air pocket, so use them! A combination of pH drop via quality pen/probe, Hanna CO2 titration test kit, and properly placed 4dKH drop checker is chefs kiss the perfect trifecta for estimating CO2 in your aquarium. And once you get the hang of it, it becomes super easy to do.

 

[nivliw]

The gist is this: the way pH probes work means that at super low KH, other acids and molecules can affect readings. For example, PO4 concentration can affect pH readings when KH is 0-1.

pH methods for determining CO2 (like hanna titration, or pH drop methods) will usually overestimate CO2 amounts when the KH is 0-1 because of this.

Not too big of a deal; I had no problem aiming for 50ppm CO2 with the hanna kit when running a zero KH aquasoil system. Was it 50ppm CO2? No. Did the test kit measure 50? yes! It worked just fine, didn't gas my fish, had plenty of CO2 available for my tank's needs.

That's part of why people underestimate drop checkers -- they function independently of KH because of the air pocket, so use them! A combination of pH drop via quality pen/probe, Hanna CO2 titration test kit, and properly placed 4dKH drop checker is chefs kiss the perfect trifecta for estimating CO2 in your aquarium. And once you get the hang of it, it becomes super easy to do.

What do you think about using a pH meter not necessarily to estimate the absolute CO₂ concentration, but simply to verify CO₂ stability throughout the photoperiod?

For example, if I reach a pH of 5.6 at lights on and then measure 5.6 again (or very close to it) several hours later, could I reasonably conclude that the dissolved CO₂ concentration has remained stable, even if the absolute CO₂ value derived from the pH is not entirely accurate because of low KH and the influence of other acids?

In other words, even if pH-based methods may overestimate the actual CO₂ concentration at very low KH, can pH still be used as a reliable relative indicator to determine whether CO₂ is stable or drifting during the day, or would the same limitations make that approach unreliable as well?