Abstract: A method, test reagent and device usable as a test strip for detecting toxic ammonia levels in water samples such as aquarium water. The volume of the water to be tested contacted with a soda lime reagent to raise the pH to at least 10, and simultaneously contacted with a hydrophobic barrier membrane capable of allowing only ammonia gas to pass through. The membrane is coated with a pH chromogenic indicator mixture which changes color if ammonia gas passes through. The color response of the sample is compared with standard color charts to determine the toxic ammonia potential.