The Seachem Glass pH Checker is a state of the art pH checker that can be used in both freshwater and saltwater aquariums. The pH checker is affixed to the tank with included suction cups and helps to closely monitor the sensitive pH balances within the tank. A highly sensitive indicating solution comes with the device, and will change color when submerged in the water so that the aquarium owner will be alerted to any changes in the chemical balance of the tank. The pH checker makes monitoring CO2 concentrations and pH levels in planted aquariums a walk in the park. Works effectively in any size aquarium. Has a diameter of 25 mm.