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.

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.


CO2 Graph.gif CO2 chart1.png

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).

AQuili CO2 tester
ADA CO2 tester

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.


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