# What is the relationship between CO2, pH and gh/kh?



## bwingate (Aug 20, 2006)

I've been keeping a planted tank for a number of years. Since I've been in school the past two years, the tank has been on "autopilot." I've decided to take back the controls and start fiddling with the tank.

I remember that the used to be a chart of the relationship between gH and pH that would allow you to derive what your CO2 levels are. Does anyone know where I can find this chart and if it is still valid? Are there other better (easier) ways to determine CO2 levels?

Tank params:

46 gallon bowfront
55w + 96w CF lighting
Pressurize CO2
tank chemistry is unknown
EI fertilization schedule being followed based on 40g of water
Moderately planted (or, I could probably double the amount of plant matter in there, but it would take some planning)

Thanks everyone,
Bruce


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## TWood (Dec 9, 2004)

bwingate said:


> I remember that the used to be a chart of the relationship between gH and pH that would allow you to derive what your CO2 levels are. Does anyone know where I can find this chart and if it is still valid? Are there other better (easier) ways to determine CO2 levels?


It's a relationship between pH and alkalinity, also sometimes called KH.

[The Krib] CO2 & Water Hardness

Look for the link that goes here:

CO2 Chart

However, the charts have mislead a lot of people. Current thinking is that 30ppm is optimal CO2, and for chemical relationships that are beyond my understanding, it appears that if you lower the baseline pH of the water by about one full unit, then that will be where the CO2 concentration is about 30ppm.

So, if you leave your water to sit out for 24 hours and get a baseline of 7.6 pH, if you add enough CO2 to lower that to 6.6 pH then the CO2 concentration will be around 30ppm.


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## hoppycalif (Apr 7, 2005)

The chart is based on the equation: CO2 (in PPM) = 3 * KH * 10exp( 7-pH ) 
If you assume the KH remains constant when you degas a sample of the water, the equation reduces to: CO2(in PPM) = CO2(in PPM)of the degassed water * 10exp(pHdegassed-pH). So, if the degassed water has 3 ppm of CO2, and the pH drops 1.0 with CO2 addition, you have 3 * 10exp(1) = 30 ppm of CO2. Unfortunately, degassing the water sample doesn't result in a 3 ppm concentration of CO2. It could be anything from about .5 up to about 5 ppm. If it degasses for 24 hours, it will be at the lower end of that range, probably. So, that method is just another guess. It is possible to "calibrate" that test to make it much more accurate, but it takes a lot of work and time to do so.

The problems with the KH/pH table method for determining CO2 are:
A small error in pH measurement makes a big error in the amount of CO2.
The equation is based on water having no other source of acidity but carbonic acid from CO2, and no other source of alkalinity but carbonate from CO2. Our tank water isn't likely to meet that criteria.


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## bwingate (Aug 20, 2006)

HoppyCalif - So what is the preferred method of determining the proper level of CO2? I don't want to get a pH meter - it just seems like too much work and expense.

I kind of understood that based on the formula, slight inaccuracies in pH measurements, either from a faulty test kit or a faulty tester, will result in wide swings of the determined CO2 levels. I'm really just looking for a "close enough" measurement that I can use without risking killing my fish.

TWood - thanks for the link

Later,
Bruce


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## BryceM (Nov 6, 2005)

"Best way?" I'd recommend measuring your "degassed" pH. That gives you something to aim for. Then I'd _SLOWLY_ crank up the CO2 over a period of a couple of days. Pay attention to the bubble count. Keep going until the fish show a little stress, then back off by a bit. The 1.0 pH drop is fairly good as a measure, but watching the fish is more important. I've played with my 46 bow enough to know how to set the CO2 just by looking at the bubble count, but this is particular to each individual setup.


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## hoppycalif (Apr 7, 2005)

One advantage, probably the only advantage of having some black brush algae in the tank is that it is a good "CO2 meter". If you have enough CO2 new BBA tufts won't start up. So, you should then be able to slowly increase the CO2 bubble rate, a little every day or two, watch the fish very carefully for signs of distress, like loss of color, and watch to see if new tufts of BBA are still starting. It should be possible to get the CO2 bubble rate above where the BBA keeps restarting, and below where the fish are bothered by the CO2. The method guaiac boy recommended is a great way to find a starting point from where you can gradually increase the bubble rate. Once you find the "sweet spot" for CO2, memorize what the bubble rate is, and use that to return to that amount of CO2 when you need to. All of this assumes using the same method of injecting CO2 all the time.


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