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Aquariums · Calculator

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) --
Status --
0 25 - 35 target 60+
Target 25 to 35 ppm for planted growth. Under 15 is too little for anything past low-light plants; 15 to 25 is fine for a low-light tank. Above 35 fish start showing stress, above 45 they can die. 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. Under 15 ppm plants stall; 15 to 25 is enough for a low-light tank. Above 35 ppm fish start showing stress (gasping at the surface, sitting in the flow), above 45 ppm they can die.

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 reliable reading. Use this calculator as a starting point, the drop checker as the source of truth.

Related tools

  • Lighting - Check if your light level matches CO2 injection.
  • Dosing - Calculate nutrient doses for a planted tank.

Further reading