Granular Ferric Oxide. Most people here buy it from Bulkreefsupply. It runs in a reactor with a pump and you want a nice slow tumble. It essentially strips phosphates out of your tank. Replace it once every month or 1 1/2 months to maintain 0 phosphates. It is pretty much one of those essential items to run in your tank as PO4 gets added constantly through feeding the fish or leaching out of the live rock. Algae blooms of this nature tend to feed off phosphates which is why we want it below .03ppm ideally.
Bulkreefsupply also sells a dual reactor which is very handy as it can run off one pump. One side is for GFO and the other is for carbon. Carbon will keep your water look more prisitine. If you do run carbon, just make sure it is rinsed very well and packed in the reactor so it is not able to tumble at all. The dust can cause problems for fish.
Also, do you use RO/DI water??
