Lil' Alert Mates are innovative color devices for continuously detecting & monitoring toxic free ammonia. They work just like Ammmonia Alert but are designed to appeal to the young aquarist. A sensor changes reversibly from yellow to green to blue, relative to the ammonia concentration. No test kits, chemicals, or procedures are needed. The device detects less than 0.05 mg/L (ppm) free ammonia & alerts you to the 1 killer before any sign of stress. It lasts over a year & replacement sensors are available. Marine or freshwater use.