A.R.M. EXTRA COARSE Calcium Reactor Media 8 lbs. Caribsea. Calcium reactors have evolved into several types that require different media sizes. We now offer our original ARM™ fine for down-flow, slow up-flow, and fluidized bed type calcium reactors and our ARM™ coarse for rapid up-flow type reactors. No matter the reactor, CaribSea has you covered.