Free Aquarium CO2 Calculator
Determine dissolved carbon dioxide (PPM / mg/L) from pH and Carbonate Hardness (KH) to optimize aquatic plant growth and prevent fish asphyxiation.
🧪 Water Chemistry Parameters
📊 Dissolved CO2 Concentration
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The pH/KH/CO2 Equilibrium in Planted Aquariums
When carbon dioxide gas dissolves into water, it reacts with water molecules to form carbonic acid ($H_2CO_3$), releasing hydrogen ions ($H^+$) and lowering the pH. Carbonate hardness (KH) acts as a chemical buffer, neutralizing acids. The mathematical relationship between dissolved $CO_2$ (mg/L), carbonate hardness (dKH), and pH is described by:
The 1.0 pH Drop Method
Because aquasoils (like ADA Amazonia) and driftwoods leach organic humic acids that falsely skew the standard KH chart, professional aquascapers rely on the 1.0 pH Drop Rule. Measure your tank's pH after letting a sample sit for 24 hours (fully degassed, ~3 ppm CO2). Injecting enough CO2 to depress the pH by exactly 1.0 unit (e.g. from 7.6 to 6.6) guarantees ~30 ppm dissolved CO2.
Why Drop Checkers Use 4 dKH Solution
Never put aquarium water directly into your glass drop checker bulb! Always use certified 4 dKH water with bromothymol blue indicator. The gas exchange across the air gap reacts solely with pure 4 dKH reference water, turning emerald green at exactly 30 ppm.
Frequently Asked Questions
What is the optimal CO2 range for high-tech planted aquariums?
Between 25 ppm and 35 ppm. Concentrations below 20 ppm encourage nuisance algae like Black Beard Algae (BBA) and Staghorn. Concentrations above 40 ppm cause fish to gasp at the surface and risk carbon dioxide toxicity.
Why do my fish gasp at night when CO2 is running?
Aquatic plants only consume CO2 during photoperiod photosynthesis. In darkness, plants reverse metabolism, consuming dissolved oxygen and releasing CO2. Always run your CO2 solenoid on a timer, turning it off 1 hour before lights-out and on 1-2 hours before lights-on.
How do I convert ppm KH to dKH?
Divide ppm ($mg/L ext{ CaCO}_3$) by 17.86. For example, 71.4 ppm KH equals exactly 4.0 dKH.