Sachem pH Alert 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 shifts to in pH levels between 5.4 and 8.0. No tests, no strips, just constant readings. Freshwater Only. pH Alert lasts up to 4 times as long as competing products. pH Alert is also almost half the size of competing products (1.5" x 2.5"). It's small, clear, and unobtrusive, so it won't detract from the view of your aquarium.