Built-in water pump and bleed valve Use in sump or hanging East twist off top For 10 to 75 gallon aquariums Dimensions: 14 3/4" x 3 7/8" Acro-cal calcium reactor offers a simple and convenient solution for maintaining calcium and alkalinity in your reef tank. After the initial setup, the calcium reactor requires minimal maintenance. A calcium reactor is almost a necessity for tanks heavily stocked with stony coral where calcium and alkalinity demands are high and will promote coral line algae growth. Calcium reactors provide a steady supply of calcium by using CO2 to dissolve media in the reactor and, thus, releasing calcium into your water. The effluent released also acts as a buffer to stabilize kh.