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Home » Fertilizers and CO2, Planted tank basis

How to measure co2 in a planted tank

Submitted by on April 24, 2009 – 5:52 amNo Comment


There is a relation between pH, KH and CO2. When we add CO2 to water, it forms carbonic acid, which lowers the pH. The more CO2 that gets dissolved into the water, the lower the pH.
Working against the CO2 to raise the pH is the KH (the Carbonate Hardness). Assuming a constant amount of CO2, a higher KH, will result in a higher pH.

Over the years, aquarists have found ways to use this relation to calculate the amount of CO2 dissolved in water. Unfortunately, it is known that other buffers such as pH-UP, pH-Down, Discus Buffers and Phosphate will throw off this relation making most test kit and calculation methods inaccurate. Most buffers can be avoided but unfortunately, there will/should always be a certain amount of phosphate in our planted tank.

CO2 table

A well known technique is to use accurate pH and KH test kits and to read off the result on a CO2 table. It is cheap and easy to do but unfortunately, other buffers present in the water makes it inaccurate. If you chose to use this method, remember to avoid using buffers and that phosphates will have an effect on your results.

Here it is:

co2 chart, table

Drop Checker

co2 drop checker Another way of measuring CO2 is to use a drop checker. This technique became popular because it allow you to monitor CO2 24/7 by simply looking at it. Additionally, buffers won’t have an effect on results. In my opinion, this is probably the most accurate way to measure CO2.

Basically, a drop checker is a reservoir that holds a 4 KH solution and a pH indicator. The reservoir is build so that there is an opening that allow gas (CO2)

exchange between the solution and the water in the aquarium.
As the CO2 gasses off from the aquarium into the drop checker, the ph lowers and the color changes (because of the pH indicator).

It is now easy to know what the CO2 level in the tank is because:
1) The pH in the checker is the same as the pH in the tank.
2) We know the solution in the checker is 4 KH
3) There is no other buffer to make our reading inaccurate.

A Green color indicates a pH of 6.6 which tells us we have +/- 30ppm of CO2 in our tank water.

Come back tomorrow for more details about Drop Checkers.

Article written by Patrice Lapointe for

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Related Posts

  1. pH Drop Checker And How To Use It
  2. Calculating fertilizer solutions for the planted tank
  3. Co2 in Planted Aquarium
  4. How much CO2 in planted aquarium?
  5. Substrate materials for the planted aquarium

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