Carbon dioxide

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Carbon dioxide (CO2)

Also called Carbonic acid gas, Carbonic anhydride, dry ice (when solid).

This is a gas given off by all plants at night. CO2 is absorbed by plants during photosynthesis.

  • CO2 dissolves quite easily in water but is also easily removed by water agitation.
  • If you wish to grow plants well in an aquarium you may need to add CO2 to the water.

It is often considered that a CO2 value of 15ppm is required in the aquarium to grow plants successfully.

Measuring CO2 in your tank

There are several methods of measuring the level of carbon dioxide in the water.

  • The most popular way is by the measurement of the pH and KH values of your tank water then you can find out the level of dissolved CO2 by reading from a graph. This method will yield inaccurate results if there are any acids other than carbonic acid in the water.


CO2 Graph.gif CO2 chart1.png

  • Excel Spreadsheet of CO2 Chart is available here.

  • By the use of a CO2 glass bubble (also called a CO2 indicator or CO2 bubble tester) submerged under water.

This glass bubble contains a small amount of liquid chemical (a 4 KH reference liquid containing Bromothymol blue) and you place in it a few drops of distilled water and then submerse it and attach it to the inside of your tank. The liquid turns either blue for too little CO2, green for the ideal amount and red too much. One end of the bubble is open to the water and so CO2 in the water disperses slowly into the chemical therefore displaying the colour. (Process takes about 60–120 minutes).

  • Note don't place aquarium water in the bubble if you have high phosphate levels or wood in your aquarium as these lower the pH and therefore will lead you to read higher levels of CO2 than there are in the water.
AQuili CO2 tester
ADA CO2 tester

  • By using a commercial CO2 test kit.

There are many of these test kits on the market. Some of these kits use the pH-KH relationship described above and so are subject to the same inaccurate readings if acids other than carbonic acid are present in the tank water. See The Krib on CO2 Test kits for more information on how they work.


  • Read the Yeast article. and its links for CO2 generation using Yeast.