Guide For Using A Co2 Drop Checker In A Planted Tank

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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. This article will outline the method to use a drop checker as well as recommend you one of best drop checker kit on the market.

The mechanism to CO2 measurement

There is a relation between pH, KH (Carbonate Hardness) 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. 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. A well known technique is to use accurate pH and KH test kits to read off the result on a CO2 table.


This method is cheap and easy to do but unfortunately, other buffers present in the water makes it inaccurate. Luckily, 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.

What is a CO2 drop checker test kit?

Change color of CO2 indicator solution

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 your aquarium water. As the CO2 gasses off from the aquarium into the drop checker, the pH lowers and the pH indicator changes its colors. By looking at the colors of the indicator and searching a color chart included in the drop checker kit, you can easily know how much CO2 (mg/l or ppm) there is in your aquarium.

A complete drop checker kit will come with these components:

  • A glass drop checker with suction cup,
  • A solution with a fixed KH (usually 4 KH),
  • A dark blue indicator (pH reagent) that changes its colors according to pH,
  • A tool (pipette or syringe) to suck and release solution,
  • A CO2 color reading chart.

! Important note: Some drop checkers on the market don't include a 4 KH solution and you have to make or buy one. Following is a drop checker kit that comes with all things you will need for the setup.

How to use a drop checker?

how to use a drop-checker

Turn the drop checker upside down and fill it with 4 KH solution and add 3 drops of blue indicator. Filling a drop checker can take a little practice. Tilt the checker slightly so the solution runs into the ball. Try to fill the ball half full, as this ensures the maximum surface area for gaseous exchange.

Turn the drop checker back over, and insert into the aquarium, making sure there is an air gap. At this point, the solution in the checker should be blue. Fixing the drop checker in a proper place under water in your planted tank with the suction cup.

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 from blue to green to yellow.


By looking at the colors of the indicator and searching a color chart included in the drop checker kit, you can easily know how much CO2 (in ppm or mg/l) there is in your aquarium.

  • Green colors indicate you have a CO2 level of 26 – 35 ppm. 30 ppm is recommended for the best plant growth but maintaining a comfortable environment for the fish.
  • Blue colors indicate you have a CO2 level of 25 ppm or less. This means the CO2 injected is insufficient and you need to slightly adjust your CO2 regulator to get green colors.
  • Yellow colors indicate you have a CO2 level of 36 ppm or more. Your tank has an excessive CO2 level and you should decrease it.

How much CO2 for planted tank?


A highly demanding aquarium plants need approx 15-30 ppm (or mg/l) CO2 to thrive, and a medium need in CO2 is 10-15 ppm (follow the guide from Tropica).

A CO2 concentration of 30 ppm (part per million) is safe for most fish and inverts and the toxicity threshold can vary considerably, depending on dissolved oxygen (O2) levels (follow Practicalfishkeeping).

Basing on these facts, the optimal CO2 concentration for both plants and fish is around 30 ppm. Many successful aquarists also maintain the CO2 level in their planted tanks at 30 ppm.

Best CO2 drop checker test kit

There are so many drop checkers on the market; however, following is the best one in my opinion. It includes three main components:

Glass CO2 Drop Checker – CO2 Monitor Kit for Aquarium Tank – Functional...
  • Complete Kit: Our aquarium water C02 drop checker/monitor contains everything you need for efficient monitoring of CO2 levels in your tank. The fish tank C02 drop checker/monitor includes the highest...
  • Easy To Use: The user-friendly CO2 detector is very convenient and gives you reliable reading in about 2 hours without syringes or other such items. Just fill the carbon dioxide monitor drop checker...
  • Clear Readings: If the pH solution turns green/light green that means your aquarium tank has the optimal level of carbon dioxide. The yellow color of the aquarium kit liquid means the CO2 level in the...
  • The glass drop checker is attached to a suction cup for easy setup,
  • The 4 dKH solution bottle (60 ml, lab verified dKH) is premixed with pH reagent,
  • The color reading chart with quick using guide that is very easy to understand and follow.

The NilocG Aquatics drop checker test kit is intelligently designed for the easiest use. The 4 dKH solution is already mixed with pH reagent and ready to use without adding anything. The flip top solution bottle is very easy to use without the need of using a pipette or syringe. The 4 dKH solution needs to be changed at least once a month. When you use up the 4 KH solution bottle, you can easily buy online.

NilocG Aquatics Co2 Drop Checker Solution 4dKH/Ph Reagent for The Most...
  • WHY SHOULD YOU PURCHASE THIS INDICATOR SOLUTION: The biggest reason is that it comes with 4dKH fluid and ph reagent PRE-MIXED. If you aren’t using 4dkh fluid your reading is very unlikely to be...
  • IMPORTANT- MUST READ- It is very important to use 4dkh fluid and ph reagent in your Co2 drop checker. If your drop checker directs you to use tank water then it will not be accurate and you will not...
  • Highest Quality 4dKH/PH solution. Monitoring co2 levels in your planted aquarium is very important for the health of your plants and fish.

Affiliate links & Images from Amazon Product Advertising API. Last update on 2022-09-11.

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1 thought on “Guide For Using A Co2 Drop Checker In A Planted Tank”

  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


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