CO2 testing

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What is it?[edit | edit source]

When you have a planted tank it is necessary to know if your plants are receiving the correct level of carbon dioxide (CO2) in the water. Too little and you get slow or dying plants and too much can cause stress and suffocation for fish.

Measuring CO2 in your tank[edit | edit source]

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 carbon dioxide by reading it from a graph. This method will yield inaccurate results if there are any acids other than carbonic acid in the water.

Graph[edit | edit source]

CO2 Graph.gif


  • By the use of a CO2 glass bubble (often called a CO2 indicator) held 24 hours submerged under water.

This glass bubble contains a small amount of chemical (usually Bromothymol blue) in a reference solution with a KH of 4. The solution is colour sensitive to CO2 from the air. You place in it a few drops of your tank 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 yellow too much. One end of the bubble is open to the water and so CO2 in the water disperses into the chemical (takes about 40mins to work).

After a week or so, organic material or bacteria gets into the test liquid and spoils the colour (it may turn the liquid yellow). You simply replaced the liquid.


  • 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.

Links[edit | edit source]