Seachem Laboratories - pH Alert . pH Alertis a unique color device designed to be placed in the aquarium or filter and monitor pH continuously. A sensor changes color reversibly to alert you shifts to in pH levels between 5.8 and 8.6. No tests, no strips, just constant readings. Freshwater Only. Simply place pH Alert visibly anywhere in the aquarium or filter. It's that easy. No chemicals or test procedures are required. When first put in service, response time will be about 30 minutes. Thereafter, pH shifts will register within ten minutes. The life of the pH Alert sensor may vary depending on a variety of tank parameters. A replacement sensor is included to ensure effectiveness for 3 to 6 months. Sensor exhaustion is indicated by a visible fading or washing out of sensor color.