Filling a CO2 Drop Checker

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 solution, 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 solution 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.