A CO2 Drop Checker is a small glass device filled with a pH indicator solution (typically bromothymol blue in a 4 dKH reference solution) that changes colour in response to the CO2 level in the surrounding aquarium water. By diffusing CO2 through the air gap between the indicator solution and the aquarium water, the checker provides a continuous, real-time visual signal of CO2 concentration without requiring any test reagents or electronic sensors.
The classic three-colour reading system makes interpretation straightforward: yellow indicates excess CO2 (above 40 ppm, potentially stressful to fish), green indicates optimal CO2 concentration (approximately 25–35 ppm), and blue indicates CO2 deficiency. This allows aquarists to dial in their regulator bubble count to maintain the ideal CO2 level for plant growth without over-gassing the tank.
The drop checker is positioned inside the aquarium, typically near the midpoint of the tank and away from direct current, to sample representative water CO2 levels. It responds with a delay of 30–60 minutes relative to actual CO2 changes, so it shows a rolling average rather than instantaneous values — making it a reliable guide for stable CO2 management.
