A water salinity control apparatus for a marine aquarium includes a filter section in which the water from a utility water supply is purified and deionized. A reservoir section includes a purified water reservoir for purified water, a salt-mixed water reservoir in which marine water at the setpoint salinity is stored, and a salt concentrate reservoir which contains fully salt-saturated water. A pump section includes pumps for pumping water from the salt-mixed water reservoir into the aquarium and for pumping water from the purified water reservoir to the aquarium. A control system adjusts the salinity of the water in the salt-mixed water reservoir and the salinity of the water in the marine aquarium. The control system receives input from salinity measuring probes that measure the salinity in the salt-mixed water reservoir and the salinity of the water of the marine aquarium.
