AquaMaxx calcium reactors are a cheaper solution, but also a very nice quality reactor for dosing alkalinity and calcium.  Using a calcium reactor on your reef tank system will allow you to dose all day 24/7, and provide stable parameters throughout the day.  Using a calcium reactor required generally a PH controller and probe, CO2 regulator and tank, and calcium reactor media.  We recommend the course media when using any style of calcium reactor as its less likely to break down if the PH drops too low.