The Alert Combo combines the pH Alert & the Ammonia Alert into one pack. pH Alert A unique colour device designed to be placed in the aquarium or filter and monitor pH continuously. A sensor changes colour reversibly to alert you to shifts in pH levels between 5.8 and 8.2. No tests, no strips, just constant readings. Freshwater only. Lasts 3 to 6 months 4 times longer than competing products, also half the size. Ammonia Alert An innovative colour device for continuously detecting and monitoring toxic free ammonia. 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) and alerts you to the NO.1 killer before any sign of stress. It lasts over a year and is for Freshwater use only.