Both freshwater fish keepers and those who use deionized/reverse osmosis systems often rely heavily on this meter. The freshwater hobbyist will appreciate the instant readings for precise hardness. The saltwater hobbyist will use the PINPOINT Conductivity Monitor™ to assess the purity of deionized and reverse osmosis water. Given proper care, the PINPOINT Conductivity Monitor™ Should last a lifetime. PINPOINT Conductivity Probe is permanent and may never require replacement. Powered by a standard 9-volt battery (not included).