CO2 / pH / KH calculator
Estimate dissolved CO2 in your planted tank from pH and KH readings.
Dissolved CO2 from pH and KH
Take a steady mid-photoperiod reading. Tank pH swings with the CO2 cycle.
If your kit reports KH in ppm CaCO3, divide by 17.86 to get dKH.
Dissolved CO2 (ppm)
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Status
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0
25 - 35 target
60+
Target 25 to 35 ppm for planted growth. Below 20 stunts plants. Above 40 stresses fish, above 50 starts killing them.
Verify with a drop checker, the math assumes a closed carbonate buffer.
How it works
The standard pH/KH/CO2 relationship assumes a closed carbonate buffer system. In a tank running CO2 injection that's roughly true.
The formula:
CO2 (ppm) = 3 * KH (dKH) * 10^(7 - pH)
Target range is 25 to 35 ppm for growth. Above 40 ppm starts stressing fish, above 50 ppm starts killing them.
Caveats
The relationship breaks down if your KH is being affected by something other than carbonates (organic acids, phosphate buffers, certain substrates). If you've added pH-down products or use peat, this number lies.
A drop checker with a 4 dKH reference solution gives you a more honest reading. Use this calculator as a starting point, the drop checker as the source of truth.