When using granular ferric oxide media, abbreviated GFO, in your reef aquarium to control phosphates, make sure not to overuse the media. GFO is great at removing phosphate from the water at a very quick pace, but if the PO4 concentrations drop too quickly, hard corals like Acroporids can be negatively affected. The coral in our aquariums adapt to all of the compounds that make up seawater, and any quick fluctuation will cause the coral to become stressed. When using the GFO media, also monitor the phosphate concentrations with digital meters to ensure the drop doesn’t happen too quickly. You may have to kill water flow to the media reactor housing the GFO every couple of days to accomplish this.