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The CO2 content in a planted aquarium should be monitored. For this task, a permanent CO2 test is used (usually referred to as drop checker). The drop checker is filled with a CO2indicator fluid, which shows the CO2 content of the water by means of a color change. Blue indicates too little carbon dioxide, yellow means there is too much, and green tells you the CO2 content is just right. Please note that the indicator fluid needs several hours to react, so the corresponding color change is not noticeable right away.
The indicator fluid should be replaced every 4 to 6 weeks in order for the drop checker to work properly. Replace the fluid at the latest when it starts losing color and turns more transparent.
Most common glass drop checkers are composed of a hollow sphere that is filled with indicator liquid and a curved tube that connects the glass sphere with a funnel-shaped inlet, on which a suction cup can be mounted. With that suction cup the drop checker can be easily stuck to the aquarium glass anywhere inside the tank. When installing the drop checker, please make sure that its inlet points downwards at all times. The funnel needs to be filled with air, not water. The CO2 from the aquarium water will diffuse through this buffer of air later on and subsequently react with the CO2 reagent in the sphere. Should the indicator fluid get into direct contact with the aquarium water, the color change will happen immediately in most cases, however, the result is usually falsified since the carbonate hardness of the aquarium water influences the indicator fluid - so please avoid any direct contact of the fluid with water.
A plastic pipette is the ideal tool for filling and emptying the sphere. It is flexible enough to reach the hollow glass ball through the curvy neck of the drop checker.
Hold the drop checker with the inlet pointing downwards for filling. The suck some reagent into the pipette and use it to fill the liquid into the sphere.
Apart from the standard drop checkers made of glass we've mentioned above, other variations are available, too.
The so-called hang-on checkers are just hung over the rim of the aquarium glass without using a suction cup. These are especially good for open pool tanks (i. e. tanks without glass braces). Due to its design, even less technical equipment will be seen inside the aquarium. Important: the opening needs to be inside the aquarium.
The Double Check v2 from Cal Aqua Labs is equipped with two spheres. The upper one works conventionally, whereas the bottom one is closed off entirely. It is filled with a reference liquid and makes color comparison and the adaptation of the CO2 content even easier.
Spherical designs such as the Orb Checker are very simplistic. They have an inside opening. Their design is even more compact and the reagent can be read especially easily.