Aquarium CO₂ & Planted Tank Calculator
Calculate dissolved CO₂ levels in planted aquariums using the pH and KH relationship. Find the correct CO₂ dosing target for healthy plant growth without harming fish.
Planted Aquarium CO₂ Guide
The pH-KH-CO₂ Relationship
In aquariums, the carbonate buffering system links pH, KH (carbonate hardness), and dissolved CO₂. The relationship: CO₂ (mg/L) = 3.0 × KH × 10^(7 − pH). This formula assumes CO₂ is the only acid in the water — accuracy decreases if other acids (phosphoric, organic acids) are present. Ideal planted tank CO₂: 20-30 mg/L. Below 10 mg/L: plants cannot photosynthesise efficiently. Above 40 mg/L: risk of fish stress and death. A KH of 5 and pH of 7.0 gives CO₂ ≈ 15 mg/L — borderline adequate for plants.
Injecting CO₂ for Plant Growth
Planted aquariums with medium to high light require CO₂ supplementation to prevent deficiencies. Three methods: pressurised CO₂ (cylinder + regulator + diffuser) — most reliable and controllable, recommended for serious planted tanks. DIY fermentation (yeast + sugar in a bottle) — cheap and effective for small tanks, but hard to control CO₂ levels precisely. Liquid CO₂ (Seachem Excel, Easy-Life EasyCarbo) — a convenient supplement, but provides only a small fraction of what pressurised systems d
KH and pH Stability
Carbonate hardness (KH) buffers the aquarium against pH swings. Low KH (under 3 dKH) means pH can fluctuate dramatically throughout the day as plants consume CO₂ during the photoperiod and CO₂ builds up at night. Minimum KH for a stable planted tank: 3-4 dKH. If your tap water has very low KH, add sodium bicarbonate (baking soda) to raise it — 1 teaspoon per 100 litres raises KH by approximately 4 dKH. For a low-pH CO₂ tank: avoid raising KH too high as this makes achieving the target CO₂ pH har
CO₂ Safety for Fish
Signs of CO₂ overdose in fish: gasping at the surface (seeking oxygen), rapid gill movement, erratic swimming, loss of colour, gathering near the filter outlet. CO₂ above 40 ppm is stressful for most fish; above 50-60 ppm can be fatal. Safe approach: start CO₂ 1-2 hours after lights on (plants consuming CO₂ and producing O₂ rather than consuming O₂). Stop CO₂ 1-2 hours before lights off. Never run CO₂ at night. Install a drop checker (colour indicator) or pH controller with an automated shut-off
Recommended for this calculator