Aquatic Aquarium CO2 Glass Drop Checker, CO2 Indicator Solution Drop Checker Solution, Fish Tank Carbon Dioxide CO2 Monitor, Co2 Test Kit for Aquariums, CO2 System, Aquarium TankMeasures CO₂ levels in water. Uses a chemical solution that changes color after 1 hour. Yellow = Too much CO₂ Green = Sufficient CO₂ Blue = Insufficient CO₂ Easy visual way to monitor CO₂ for aquariums or experiments.