Aquarium CO₂ & Planted Tank Calculator

Planted Aquarium CO₂ Guide

What's the key thing to understand about 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.

What should I know about 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, such as Seachem Excel or Easy-Life EasyCarbo, is a convenient supplement, but provides only a small fraction of what pressurised systems deliver, and works best in low-tech or lightly planted tanks rather than as a genuine substitute for a full pressurised CO₂ system in a heavily planted setup.

What should I know about 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 harder, since a higher KH buffers the water more strongly against the pH drop CO₂ injection is meant to produce.

What should I know about 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