Carbon dioxide (CO₂)
The single nutrient that decides whether a planted tank lights up or limps along. Calculated from pH and KH, dangerous when over-injected.
What it is
Dissolved carbon dioxide. Plants pull it from the water column to drive photosynthesis; fish exhale it as a metabolic waste. In a non-injected tank, CO₂ sits at the equilibrium with the atmosphere (~3 mg/L) — enough for slow-growing low-tech plants, not enough for the demanding stems, carpets, and red species that planted-tank hobbyists are usually after.
CO₂ injection rigs push pressurised gas through a diffuser; the dissolved concentration is what matters, and it's not what the rig flow rate tells you.
Why it matters
The difference between 5 mg/L CO₂ and 25 mg/L CO₂ is the difference between a tank that grows algae and a tank that grows plants. CO₂ is the limiting nutrient in 90% of "I dose everything but nothing grows" cases.
It's also the parameter most likely to kill fish suddenly. Above 40 mg/L sustained, fish can't unload metabolic CO₂ across their gills into water already saturated with it, and they suffocate even though oxygen is plenty.
How to test
Three options. A drop checker (4 °dKH solution + bromothymol indicator) lags by 24 hours but reads honestly — yellow = too much, green = target, blue = not enough. A pH probe + KH reading runs through the pH + KH formula; the free guide explains the chemistry and when the formula lies. A direct CO₂ meter is most accurate but expensive.
What high and low look like
Below 10 mg/L: slow plant growth, algae bias toward green hair and brown diatoms. 15–30 mg/L: target band for most planted tanks. Above 35 mg/L: fish gasping at the surface, particularly at lights-off when the photosynthetic O₂ production stops.
How to fix
Too low — add or increase CO₂ injection; aim for a 1.0 pH drop from non-injected baseline. Too high — back off injection, increase surface agitation (CO₂ off-gases through any turbulent surface), and run an air stone on a timer for the dark period as a fail-safe.
