Easy CO2 Calculator!

  • Get the NEW AquariaCentral iOS app --> http://itunes.apple.com/app/id1227181058 // Android version will be out soon!


2009 VMAX
Original poster
Jul 6, 2002


AC Members
Apr 17, 2001
Ghent, Belgium
Thanks for the link ! I do however have some questions on some comments that are made on that site :

Quote 1 : "This calculator (and the chart based on this formula) will only work if your water is carbonate buffered. If your water contains high levels of phosphates, it will alter your water properties, and invalidate these CO2 calculations."

How do phosphate alter these calculations :confused:

Quote 2: "If you aren't adding CO2 to your water, and the CO2 level based on the pH and KH indicates more than 5ppm, then it is very likely that some other buffer (such as phosphate) is present in your water. In an inhabited aquarium, the amount of CO2 produced by the fish will not have an effect on CO2 levels in the water. Any excess CO2 created by fish will dissipate into the air, leaving a fairly constant CO2 level of about 3-4ppm. If you test your pH and KH, and without adding any CO2, the chart says you've got 20ppm CO2, don't believe it. "

Does this mean that without adding CO2 with DIY or a pressurized system, I cannot have concentrations over 5 ppm ? Why would my CO2 concentration in the water increase by adding pressurized CO2, and NOT by adding more fish ?

-------> :confused:

Anyone who understands the above quotes/questions, is invited to share his/her knowledge ...


AC Members
Oct 5, 1998
Braddock Heights, MD
The site referenced is a good one.

Quote 1: The CO2/KH/PH conversion tables or formulae are based on carbonate/bicarbonate buffering (that present in most natural waters), not phosphate or others. CO2 and the carbonate/bicarbonate buffer system interact directly. If other buffering is present, the pH and KH values will be different from carbonate/bicarbonate only, and the table or formulae cannot be used to read CO2.

Quote 2: The equilibrium with atmospheric CO2 is approx. 2-5 ppm CO2 (varying with elevation above sea level). The lower reading there is from high in the Rockies - if you live higher, you could get lower. On this one I differ from Chuck's conclusions a bit. I suspect (but have not asked) that he is giving an average figure, assuming certain levels of water disturbance from HOB filters, etc., and moderate fish loads. With filtration handled carefully, good subsurface water circulation but no surface disruption, and still with moderate fish loads, you can get above 5ppm CO2, certainly at the end of the dark cycle. I agree with him that 20ppm CO2 is unreachable with such techniques. Adding more fish will certainly increase CO2 a bit, but will increase NH4, phosphates, general pollution even more, and decrease O2 to levels requiring added aeration, blowing off CO2 - it quickly becomes a lose/lose situation. Overcrowded plant tanks are a no win situation from what I've seen on the boards.