The pH/KH/CO2 Relationship

Here is a rough table to help you determine how much CO2 is present in
your aquarium water based on accurate measurements of pH and KH.


Carbon Dioxide, (CO2) is a vital plant nutrient, and must be present in sufficient quantity in your aquarium for your plants to thrive. In order to use the chart below to measure the amount of CO2 in your tank, first measure the pH and the carbonate hardness in your tank, locate the values on the chart below, and where the two values intersect you will find the amount of CO2 in your tank. If the pH and Carbonate hardness measurements differ from those on the chart, use the values nearest.

The amount of CO2 present is expressed in milligrams per Liter. The green area represents CO2 optimum levels for the aquarium. High CO2 levels, yellow area, usually do not affect the plant life but are dangerous to fish. A planted aquarium requires about 1 gram of CO2 for every 25 gallons (100L) of water every day. So you will want a measurement of between 10 and 40 milligrams per quart or Liter.

The CO2 pH/KH relationship is a good guideline to get a generally reading of C02 levels, but it not exactly perfect. It assumes tested aquarium water has perfectly set levels of KH and pH which don’t vary. The ph/KH/CO2 Chart can help beginners gain a basic understanding of how much C02 they are injecting. It’s been a great tool that has taught many hobbyists how the acid affects pH levels, and how buffering KH can prevent pH from falling drastically. But in the end, it can only provide one a general gauge of their CO2 levels. A C02 drop checker is one of the best way to monitor your CO2 level.