The relationship is based on the carbonate-bicarbonate equilibrium in water. Dissolved CO₂ forms carbonic acid, which dissociates into bicarbonate and carbonate ions. The balance between these species determines pH. For aquarium use, we use the simplified formula: CO₂ (ppm) = 3 × KH × 10^(7−pH), where KH is measured in German degrees (dKH). This formula assumes carbonate is the primary buffer — it works well for most freshwater planted aquariums. Note that other buffers (phosphates, tannins, etc.) can affect accuracy.

The widely accepted ideal CO₂ range is 15–30 ppm. Within this range, aquatic plants receive enough carbon for vigorous photosynthesis and healthy growth. At 20–25 ppm, most demanding plants (like carpeting species) thrive. Below 10–15 ppm, plant growth slows significantly and algae often becomes problematic. For tanks with sensitive fish or shrimp, staying near the lower end (15–20 ppm) is prudent. High-tech tanks with CO₂ injection often target 25–30 ppm, but careful monitoring is essential to avoid stressing livestock.

The pH/KH/CO₂ table method provides a reasonable estimate for most freshwater aquariums, but it has limitations. Accuracy depends on carbonate being the dominant buffer. If your water contains significant phosphates, humic acids (from driftwood or peat), or other buffering compounds, the estimation may overstate or understate actual CO₂ levels. For precise measurement, a drop checker with a 4 dKH reference solution is recommended. However, this calculator remains a valuable quick-reference tool used by aquarists worldwide for routine monitoring.

Most aquarium fish tolerate CO₂ levels up to 30 ppm without issues, provided there is adequate oxygenation. At 30–40 ppm, sensitive species (like some tetras, discus, or loaches) may show signs of respiratory stress — rapid gill movement, gasping near the surface, or lethargy. Above 40 ppm is dangerous for nearly all fish and can be lethal. Shrimp and snails are particularly sensitive; for invertebrate-heavy tanks, keep CO₂ below 20–25 ppm. Always observe livestock behavior when adjusting CO₂, especially during the first few hours after changes.

KH (carbonate hardness) is typically measured using a liquid test kit — the most common and affordable method. The API KH Test Kit is widely used: add drops of reagent to a water sample until the color changes from blue to yellow, with each drop representing 1 dKH. For more precision, digital titration devices or photometers are available but cost significantly more. Test strips also exist but are generally less accurate. Measure KH regularly, as it can deplete over time in planted tanks with CO₂ injection. If KH drops below 2–3 dKH, pH may become unstable (pH crash risk).

No, this calculator is designed for freshwater aquariums only. Saltwater has a much more complex buffering system involving borates, magnesium, and other ions that render the simplified pH/KH/CO₂ relationship inaccurate. Marine aquariums typically maintain a stable pH of 8.0–8.4 through alkalinity management, and CO₂ estimation requires different methods. For reef tanks, focus on alkalinity (dKH) measurements and use a pH probe for monitoring rather than relying on CO₂ estimation tables.