Aquarium CO2 Calculator
Calculate CO2 concentration in your aquarium from pH and KH values.
Shows if levels are safe for fish and optimal for plants.
Aquarium CO2 Concentration can be calculated from the relationship between pH, KH (carbonate hardness), and dissolved CO2. This is essential for planted aquariums where CO2 supplementation promotes healthy plant growth.
The Formula:
CO2 (mg/L) = 3 × KH (dKH) × 10^(7 - pH)
This formula is derived from the carbonate equilibrium chemistry of water. It relies on the fact that CO2 dissolved in water forms carbonic acid, which affects pH in a predictable way based on the buffering capacity (KH) of the water.
What is KH? KH (carbonate hardness) measures the concentration of carbonate and bicarbonate ions in water, expressed in dKH (degrees of carbonate hardness). 1 dKH = 17.86 mg/L of CaCO3. Typical freshwater aquariums have KH between 2 and 12 dKH.
What is pH? pH measures how acidic or alkaline the water is on a scale of 0 to 14. Most freshwater aquariums maintain pH between 6.0 and 8.0. Adding CO2 lowers pH because dissolved carbon dioxide forms carbonic acid.
Target CO2 Levels:
- Below 10 mg/L: Too low for most aquatic plants. Growth will be slow.
- 10 to 20 mg/L: Adequate for low-demand plants like Java Fern and Anubias.
- 20 to 30 mg/L: Optimal range for most planted tanks. Good growth without stressing fish.
- 30 to 35 mg/L: Upper limit. Some sensitive fish may show stress.
- Above 35 mg/L: Dangerous. Fish may gasp at the surface, become lethargic, or suffocate.
Tank Volume: The calculator accepts tank volume in both gallons and liters. 1 US gallon equals 3.785 liters. Tank volume itself does not affect the CO2 concentration formula, but it helps you understand how much CO2 gas you need to inject to reach your target level.
Practical Tips: Measure pH and KH at the same time of day for consistent readings. CO2 levels fluctuate throughout the day — highest at lights-on (after overnight buildup) and lowest before lights-off (plants have consumed CO2 all day). Use a drop checker with bromothymol blue indicator solution as a visual CO2 monitor: green means approximately 30 mg/L, blue means too low, yellow means too high.
Important Limitation: This formula assumes KH is the only buffer in the water. Products like pH buffers, tannins from driftwood, or phosphate-based buffers can give false readings. If you use such products, a drop checker is more reliable.