What kind of reactor? A dual chamber calcium reactor will have the ARM in one and sand in the other to buffer the PH before the water goes back into the tank. A regular ole media reactor will have carbon or Granular Ferric Oxide (GFO) for removing phosphates. I run both carbon and GFO in my single reactor; carbon on the bottom, then a sponge, then GFO, then another sponge at the top. Works great for clean water. I use 2x the amount of carbon that I do GFO and change them out at the same times, once a month.