Planted Tank

CO₂

Dissolved CO₂ from KH and pH readings.

Carbonate hardness (KH)4.0 °dKHpH6.80

Measure KH and pH from the same water sample, ideally mid-photoperiod when CO₂ injection is stable. This chart assumes carbonate alkalinity dominates KH.

Results

Dissolved CO₂

19.0ppm

Status

Planted sweet spot

Roughly 15–35 ppm is the usual target for high-tech planted tanks with livestock. Confirm with a drop checker.

Quick reference

<15 ppm — low for high-tech planting
15–35 ppm — common planted target
35–50 ppm — high; watch fish behavior
>50 ppm — reduce injection / increase aeration

How to use this

Test KH with a liquid kit (drops to color change) and convert to °dKH if your kit reads ppm CaCO₃ (divide by ~17.9).
Test pH with a calibrated meter or fresh liquid reagent — strip tests are too coarse for CO₂ charts.
Enter both readings here to estimate dissolved CO₂ from the carbonate equilibrium relationship.
Confirm with a 4°dKH drop checker (lime green ≈ target) before trusting the number alone.

Estimate used

CO₂ (ppm) ≈ 3 × KH(°dKH) × 10^(7 − pH)
Example: KH 4, pH 6.8 → 3 × 4 × 10^0.2 ≈ 19 ppm

KH 4 °dKH — what pH implies

Same KH, lower pH means more dissolved CO₂. Drop checkers use 4 °dKH solution so lime-green lines up with this chart. Formula fails when KH is near zero.

pH	CO₂ (ppm)	Reading
7.0	12	Low for high-tech plants
6.8	19	Planted low end
6.6	30	Usual high-tech target
6.4	48	High — watch fish
6.2	76	Cut injection, aerate

Worth knowing

Inject CO₂ only when lights are on; shut it off (or use a pH controller) at night when plants stop consuming it.
Surface agitation gas-exchanges CO₂ out — balance diffusion against oxygen for fish.
If KH is near zero, this formula breaks down; remineralize or use a different monitoring method.

Usual mistakes

Mixing KH and GH — only carbonate hardness belongs in this chart.
Reading pH right after a water change or with old reagent.
Chasing 30 ppm with livestock that already gasp — prioritize animal safety.

See also: Lighting · Fertilizer · Water Change