Using a CO2 drop checker test kit is the most convenient and economical method to measure the CO2 concentration in a planted aquarium. This small but very helpful equipment is an essential part of planted aquariums today because it helps you to control the CO2 levels in your planted tank – an important factor decides if your tank is successful for not. In this article, I will cover how to use a drop checker as well as review the best drop checker kit easy to use for you.

What is a CO2 drop checker test kit?

A complete drop checker test kit looks like thisA complete drop checker test kit looks like this

A drop checker is a U shaped glass container filled with a indicator solution. The reservoir is build so that there is an opening that allows gas (CO2) exchange between the solution and your aquarium water. As the CO2 gasses off from the aquarium into the drop checker, the pH lowers and the color changes. By looking at the color in the reservoir, you can easily find how much CO2 there is in your aquarium.

Using a CO2 drop checker kit in a planted tank

How to set up a drop checker?

  • drop-checker-manual
  • Turn the drop checker upside down and fill it with several drops of CO2 indicator solution.
  • Tilt the checker slightly so the solution runs into the ball.
  • Turn the Drop Checker back over, and insert into the aquarium. At this point, the solution in the checker should be blue.

After adding the indicator solution to the glass drop checker, fixing it in a proper place under water in your planted tank using the suction cup fitted on the glass checker. Waiting for at least 2-3 hours to see the color change of the CO2 indicator solution.

How do we read its colors?

Once the checker is in the aquarium, it will take a few hours before CO2 in the aquariums exchanges across the air gap. After 2-3 hours, CO2 in the checker and CO2 in the aquarium should be the same. The increased CO2 level in the Drop Checker will affect the pH which will change the color of the regent.

Change color of CO2 indicator solution

  • A healthy aquarium has a CO2 level of anywhere between 20 to 40 ppm. 30 ppm is an ideal level for both plants and fish.
  • A blue color indicates you have a CO2 level of 25 ppm (part per million) or less.
  • A green color indicates your have a CO2 level of 26-35 ppm. It is ideal if your checker changes to the green color.
  • A yellow color indicates you have a CO2 level of 36 ppm or more.

Normally, if you want to measure the CO2 concentration, you need to have an accurate measurement of pH and KH of the aquarium water then infer the CO2 level. However, with a drop checker, CO2 in the aquarium water is measured through the solution in the checker thus reduce the risk of error. So it is more convenient and economical than the method using test kits to measure pH and KH then infer the CO2 level.

Best drop checker kit for your fish tank

  • Best drop checker kit5 star rating (5/5 by editor)Check on
  • Rhinox Glass Drop Checker Kit includes:
  • A glass drop checker to contain CO2 indicator solution,
  • A suction cup to bind the glass checker to your aquarium side,
  • A 10 ml CO2 indicator solution,
  • A 2 ml syringe,
  • And a color chart for reading the CO2 level.

For using the kit, only add 5 drops of CO2 indicator solution to the glass checker, then place them inside your aquarium using the suction cup. You should wait for at least 2 hours before reading the result.

  • Rhinox Glass Drop Checker KitRhinox Glass Drop Checker Kit

  • If the blue CO2 indicator turns green, congratulation, your aquarium has the right balance of CO2.
  • If the blue CO2 indicator still remain the blue or doesn’t get the wanted green, it mean the CO2 injected is insufficient and you only need to slightly adjust your CO2 regulator to get the right green.
  • If it turns yellow, your tank has excessive CO2 and you should decrease the CO2 injected. Once ensuring the CO2 injected is relevant, you need to empty the glass checker and repeat the test until you get the green color.

You only has to change the indicator solution if it turn yellow. When you use up your CO2 indicator solution bottle, you can buy a new one here.

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1 comment

  1. My tank is less than a week old. 6.5 gallon tank with 6 glo fish Tetra. Water was clear and after adding fish water it was cloudy. Two dats later water is clear, however there are flacks floating around the aquariums PH balance tester. Why is flacks floating around aquarium?

    I’m itder