Calcium Reactors:  help to elevate and maintain calcium levels in the aquarium. It maintains your kH level. With calcium levels at or about 380 with a properly maintained kH, you will have reached the perfect level of calcium saturation. Another side benefit of calcium reaction comes from the carbon dioxide. CO2 is combined with aragonite or calcium carbonate to lower the pH just enough to allow it to dissolve. Once this occurs, the calcium saturated water is returned to the tank with trace amounts of CO2. This CO2 helps promote photosynthesis when your tank is in daytime mode. Calcium reactors rarely need to be refilled. When they do, they can be filled with aragonite.