Legend CO2 Drop Checkers are used for long-term monitoring of the CO2 levels in the aquarium. A Legend CO2 reagent is filled into those CO2 Drop Checkers and will change the color according to the amount of dissolved CO2 in the aquarium water. Green will indicate a proper CO2 value, yellow will indicate to much CO2 and blue will show not enough CO2.

Please note that the required CO2 Drop Checker reagent is sold separately and does not come with this glass CO2 Drop Checker.


1 x Legend CO₂ Checker with suction cup

Legend – CO2 Drop Checker

Height    55 mm

Width     50 mm


The permanent test is filled with 1 – 2 ml of the Legend CO2 indicator solution and then positioned in a well-circulated area in the aquarium. After about 1 – 2 hours, the CO2 content of the aquarium water can be read. A green colour indicates an optimal CO2 supply of approximately  30 mg/l.


A Legend CO2 indicator solution is very useful in an aquarium. Often, aquatic plants lack the essential CO2, preventing optimal growth. With a CO2 permanent test, the CO2 level can be controlled with sufficient accuracy. The dye bromothymol blue used has the property of changing colour in a defined pH range.

CO2 diffuses from the aquarium water through the air bubble in the permanent test into the test solution and vice versa. Depending on its CO2 content, the pH value of the test liquid and thus its colour changes. A defined carbonate hardness in the test liquid determines at which CO2 concentration a certain pH value is established. The Legend CO2 indicator solution turns light green when the pH value is around 6,6. This is achieved at a CO2 concentration of 30 mg/l. For this reason, the Legend CO2 indicator solution indicates a light green tone at a CO2 value of the aquarium water of  30 mg/l. If the CO2 supply in the aquarium is less than  30 mg per litre, the test reagent remains blue; if the CO2 supply exceeds the mark of 30 mg per litre, the test reagent turns yellow.

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Made of high-quality glass

Visually very appealing

Good to read

The necessary suction cups are included