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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.

Welcome to ScapeCrunch, @Gabi!
We would love to get to know you. Please tell us about yourself. What tank do you have?

Planted 12/17/25

Goals and Past History:

This tank has been running since July 2024 and was originally inspired by George Farmer's home display tank.

I stocked mine mainly with swords and crypts along with Hyphessobrycon wadai (blueberry tetra), H. pulchripianis (lemon tetra), corydoras pygmaeus and Amano shrimp. These are all still in the tank today, sans a few jumpers I lost. Here's what it looked like before I changed goals to a high energy stem plant tank.



Things were humming along well with this tank until March 2025 when the tank suffered a major planaria outbreak. I wanted those vile critters gone, so I dosed to full cycles of No Planaria. This product worked on the planaria, but also severely damaged the beneficial bacteria colony. Instant algae fest which I battled with for the next 6 months.

Things cleared up in September 2025 and were going well. I hadn't been on Scapecrunch for many months, so I decided to get back into the forum and see how everyone's tanks were looking. I perused many a tank journal and after seeing everyone's gorgeous tanks, I decided I wanted some color and a change! I decided to give a high energy stem plant tank a go, so here we go!

This is my first journal and first real attempt at creating a tank that's full of different colors, contrasts and textures.

Equipment, specs, plants and ferts coming soon....

Era of AI slop is truly upon us. Firstly, no one holds the Oxyguard analyzer's probe as it takes 15-20mins to get a reading.
Size of the box is wrong and no shadows below the box, caps missing and a strap that goes nowhere. Aquarium looks fake as well. Real pic below for comparison.

Tried to open the app to change a setting. Goes to a login screen. Checked Chihiros support, and it seems to be an issue with the latest version of the app. The issue appears to be widespread.

Discussion on the Chihiros site.

Inspired by the thread on CO2 controllers, and @Art experiment with modified pH probe / drop checker, I came up with what is to the best of my knowledge a new approach for our hobby.

I am not strong on the chemistry of CO2 in water, so it would be great if others chime in and comment if this approach makes sense.

Use the principle of Henry's law and fill the space under a closed aquarium lid with the correct concentration of CO2, using an affordable 65 USD sensor or similar with solenoid to release CO2 above the water. Control CO2 partial pressure so that the tank water will reach equilibrium with this gas pocket at about 30 ppm in the water. This would allow for very stable CO2 in the tank, and a big saver in CO2 as we don't rely on the outgassing via surface agitation to achieve stability. Besides that, it will be only weakly dependent on water chemistry (this may need to be tested and/or confirmed/quantified by chemists, as the chemistry and temperature do have some influence on CO2 solubility?).





Notes:

• My first thoughts/estimations are that one can reach 30 ppm in the tank water very fast, and have probably more than 90% savings on CO2 consumption. As we now measure CO2 in the gas pocket, independent of water chemistry, it would (hopefully) also be the first time that we can forget water chemistry as a major factor, complications with pH probes and using pH as a proxy, and indeed know the real water CO2 ppm more accurately and with less risks of misinterpretations of measurements.
• We don't need a precision expensive CO2 regulator anymore, no diffuser or reactor, no pH probe or drop checker. Just a sensor with electronics that switches a solenoid on and off. It would probably be good to have a simple air pump added, so that O2 and other gasses can exchange, but this would give very low CO2 losses.

From a physics and chemical perspective the approach is very similar (in reverse) to what professional dissolved CO2 probes do (see attached: measure CO2 concentration in a gas pocket that is in equilibrium with water), so that gives me some confidence that the above will work. However I am not confident that my understanding of chemistry of CO2 in water is good enough, and I may miss important aspects. Hope others chime in, and help to quantify if the chemistry is a minor or perhaps a major complication.

Attached file: Datasheet explaining the principle and use of a dedicated dissolved CO2 meter (for most in the hobby too expensive, but similar scientific principles applied as in above ideas).

I came into some Hygrophila polysperma "white" by Tropica recently. Grew it out and it seems to be able to be shaped into good midground bushes so I decided to create a layout to showcase it. Contemplated whether to use it as the only white plant in the tank, but decided to use some Anubias white petite as well so that the white polysperma doesn't stand out awkwardly.

Sorry, but I just find this hilarious! Perhaps they'll ask scientists if there's a way to turn down the brightness and photoperiod of the sun.

Headline:

"Reflecting Pool woes: Trump administration turns to hydrogen peroxide in latest bid to beat back algae"​

Hi All,

This is something I've been wanting to do for years, and I think I finally found a sensor that will work. I've always had trouble reading the API tests, and I've always been miffed that the reefers get the cool digital test readers - and wanted to take a crack at building one that could potentially read any freshwater test given a blank/known concentration as a calibration.




A sensor came out from ams (AS7343) in 2022 that unfortunately has been made EOL, but has a replacement (TCS34488M) with a similar package that might work for future versions.


I recently got my hands on a qwic version of the AS7343 sensor from sparkfun, and figured its time to put together something.

Goals:

• As cheap as possible
• Universal as possible
• Fit API glass/plastic vials (not sure yet if the plastic vials will read ok)
• Start with Nitrate/Phosphate and see if I can add more there

I figure I'll need 2 light sources (warm white, and IR for reading the phosphate test), but can use the same sensor array across most tests. I can use a small-form ESP32 as the MCU to give it USB-C power, wifi/bt connectivity if needed, and keep it small. Small/cheap ~1.3" OLED screen for displaying results/selecting tests.

Enclosure will be 3d printed.



First pass at a sketch - I might drop the screen if I can give the device a web interface though, which will make the device even smaller/cheaper, reduces the need for physical buttons on the unit, and a 2nd pcb entirely. Also not sure if it will need a cover for the top of the vial, or how much ambient light will affect the reads. TBD. Will order some XIAO ESP32S3 to play with and see how far I can take it.

I'll log progress here, and am very open to suggestions and ideas. If successful, I'll release the files so it can be easily replicated.

Hey all,

I’m setting up a new tank in Tasmania, Australia. I’ve had planted tanks many years ago in Sydney. To say the selection of equipment and plants here is limited, would be generous. While I can get lighting and filtration online, substrate is proving difficult. I’m going for a 90P (if I can get a low iron glass one here) so I need enough substrate to do the job. My options would be inert sand/gravel with root tabs or try an aqua soil of some variety. The only thing I can find locally to avoid huge shipping costs is Aqua Earth - which is stocked by a local fish supplier at about $60 AUD/6L.


Has anyone used this product before or have any reviews?

This tank is planned to have a few stem plants, various crypts, tiger lotus and hopefully a small carpet area of Monte Carlo or Marsilea and some pygmy chain swords (echinodorus).

Cheers
Chimera