Lil' Alert Mates - Barry Cuda 1 Year Monitor LM Monitors Toxic Ammonia In Freshwater And Marine Aquariums Innovative Color Devices For Continuously Detecting And Monitoring Toxic Free Ammonia; Easy To Use, Install, And Read A sensor changes reversibly from yellow to green to blue, relative to the ammonia concentration No Test Kits, Chemicals, Or Procedures Are Needed It Lasts Over A Year And Replacement Sensors Are Available