Drop checker

A drop checker is a small glass bulb, usually suction-mounted inside the tank above the waterline, that holds a mix of pH indicator solution and a reference fluid. As CO2 from the water diffuses into the air gap inside the bulb, it slowly changes the color of the indicator solution, giving an approximate read on how much CO2 is dissolved in the tank.

The color scale runs roughly from blue, meaning too little CO2 for good plant growth, through green, considered a safe working range for most planted tanks, to yellow, which signals CO2 levels high enough to stress or harm fish and shrimp. Because the indicator fluid reacts slowly, a drop checker shows conditions from an hour or more earlier rather than the current instant, so it works best as a trend check rather than a live alarm.

Most aquarists check the color once or twice a day, especially in the first few weeks after setting up CO2 injection, and adjust the regulator or bubble count gradually based on what they see. A common mistake is reading the drop checker right after turning the lights on or off, when the color has not caught up to the new CO2 level yet, which can lead to overcorrecting the CO2 dose.