Aquarium CO2 and fertiliser calculator
In a tank with no other acids, pH and carbonate hardness together tell you the dissolved carbon dioxide.
dKH
ppm
ppm
% NO3 in KNO3
Dissolved CO2
19.02ppm
- pH that would give 30 ppm at this KH
- 6.602
- Potassium nitrate for the dose
- 2.610
- Weekly dose across three doses
- 7.830
- Matching phosphate target The Redfield-inspired 10:1 nitrate to phosphate ratio.
- 1.00 ppm
- CO2 in the useful range
- 0
- Nitrate increase needed
- 8.0 ppm
Show the working
- pH = 6.8
- Carbonate hardness = 4
- Water volume = 200
- Target nitrate = 10
- Current nitrate = 2
- Nitrate in the salt = 61.3
- Dissolved CO2 = 3 * 4 * 10^(7 - 6.8) = 19.0187
- Nitrate increase needed = max(10 - 2, 0) = 8
- no3_mg = 200 * 8 = 1600
- Potassium nitrate for the dose = 61.3 > 0 ? 1600 / (61.3/100) / 1000 : 0 = 2.610 11
- Matching phosphate target = 10 / 10 = 1
- pH that would give 30 ppm at this KH = 4 > 0 ? 7 - log10(30 / (3 * 4)) : 0 = 6.602 06
- CO2 in the useful range = 19.0187 >= 20 && 19.0187 <= 35 ? 1 : 0 = 0
- Weekly dose across three doses = 2.610 11 * 3 = 7.830 34
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
Scenarios & local history
Compare a saved scenario
Recent calculations
The formula
- CO2 in ppm = 3 x dKH x 10^(7 - pH)
- the relation assumes carbonate is the only significant buffer
The pH and KH method fails badly in tanks with peat, driftwood tannins or phosphate buffers, all of which lower pH without CO2.
A drop checker with 4 dKH reference solution measures CO2 directly and is far more reliable.