It is 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 to changes in pH level between 5.8 and 8.6.Only fresh water. Simply place pH Alert ™ visibly anywhere in the aquarium or filter.So easy. No chemicals are necessary measurement procedures.When put into service for the first time the response time will be about 30 minutes. Thereafter, changes in pH was detected in about ten minutes. The life of pH Alert ™ sensor may vary depending on a variety of tank parameters. It includes a replacement sensor to ensure effectiveness of 3 to 6 months.