Ammonia Alert detects and continuously monitors free ammonia in marine or freshwater aquaria. Simply place it in the aquarium (using the provided suction cup) and read anytime. It will correctly read free ammonia levels when ammonia removers are in use. The unit consists of a waterproof reference color card and a sensor. The sensor, located in the center of the reference scale, changes reversibly from yellow to green to blue relative to the ammonia concentration. Properly maintained, the sensor has a useful life of over a year.