The Seachem One Year Alert Combo pack combines a Seachem Ammonia Alert and a Seachem pH Alert with replacement sensor to provide a years supply of testing. The Seachem Ammonia Alert and pH Alert are almost half the size of competing products. At 4 x 6.5cm, they are small, clear, and unobtrusive, so they won't detract from the view of your aquarium. Simply place the Ammonia Alert and pH Alert visibly anywhere in the aquarium or filter. It’s that easy. No chemicals or test procedures are required. Seachem Ammonia AlertThe Ammonia Alert's sensor changes reversibly from yellow to green to blue, relative to the ammonia concentration. It detects less than 0.05 mg/L (ppm) free ammonia and alerts you to the number one killer before any sign of stress. The presence of the free ammonia is detectable continuously with a response time of about 15 minutes. Response to decreasing ammonia is slower, requiring about 4 hours to go from TOXIC to SAFE on removal of ammonia. Suitable for marine or freshwater use. Seachem pH AlertThe pH Alert is a unique colour device designed to be placed in the aquarium or filter and monitor pH continuously. A sensor changes colour reversibly from yellow to orange to red alerting you to shifts in pH levels between 5.4 and 8.0. When first put in service, pH Alert's response time will be about 30 minutes. Thereafter, pH shifts will register within ten minutes. Suitable for freshwater use only.