CO2 Injection Calculator

KH measures the water's buffering capacity against pH changes, and dissolved CO2 forms a weak acid that lowers pH — the more buffering capacity (higher KH) the water has, the more CO2 it takes to push pH down by the same amount. This is exactly why the same CO2 concentration produces a different pH reading in low-KH water versus high-KH water, and why the formula multiplies KH directly into the CO2 calculation rather than treating pH alone as sufficient.

Treat it as a reasonable starting point rather than a precise final setting — bubble size and CO2 dissolution efficiency vary meaningfully between different diffusers, reactors, and tank setups, so two tanks at the identical calculated bubble rate can end up with somewhat different actual dissolved CO2. Most aquarists use this kind of estimate to get in the right neighborhood, then fine-tune the actual rate while monitoring fish behavior and a drop checker or direct CO2 test.

Fish extract oxygen from water through their gills, and elevated CO2 in the water interferes with that oxygen exchange even when dissolved oxygen levels themselves are adequate, essentially making it harder for fish to breathe. This is why planted-tank keepers running higher CO2 for plant growth typically increase surface agitation right before lights-out, allowing excess CO2 to off-gas overnight when fish are most vulnerable and photosynthesis isn't consuming any of it.

A standard pH test only tells you the current pH, not why it's at that level or what CO2 concentration is causing it — this figure inverts the KH-CO2-pH relationship to predict what pH your tank should settle at once you've reached your desired CO2 target, given your measured KH. Comparing that predicted value against what your pH probe actually reads is a useful sanity check on whether your CO2 injection is landing where you intended.