Abstract: An automatic calcium reactor preferably includes a reactor body, a carbon dioxide separator, a salt water pump, a recycling venturi and an electronic controller. The reactor body includes a lower chamber and an upper chamber. A source of calcium is placed in the upper chamber. The carbon dioxide separator is suspended near a top of the upper chamber. A salt water outlet is formed through a side wall of the reactor body. Salt water from the aquarium and salt water from the reactor body are fed into the salt water pump. A recycling venturi includes a salt water inlet, a carbon dioxide inlet and an outlet, which is connected to the lower chamber. A float switch is retained near a top of the upper chamber. Rising salt water inside the upper chamber trips the float switch and opens the carbon dioxide valve through the electronic controller.