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GFO - Grandular Feric Oxide - Do I Need It?

High phosphate levels are a recipe for disaster in saltwater aquariums. It is a nutrient for nuisance algae and also hampers the formation of calcium skeleton in corals. There are several different methods for dealing with phosphate levels but one they I like is using granulated ferric oxide (or GFO) in a media reactor. GFO is best used with a media reactor for several reasons.

First, it is a very fine media and would easily escape from any bag you put it in.

Second, it clumps together if you don't have water moving through it.

When using GFO in a media reactor watch the flow rate of the water through the media. It does not need to be very fast at all. You don't want it looking like boiling water where it is throwing up small particles in the reactor. You just want the very top inch or less turning over ever so slightly like water right before a boil. Otherwise you will get GFO dust all inside your sump and possibly inside your display tank. Depending which reactor you choose, it is a good idea to run the output of the reactor into a filter sock. Worst case scenario some GFO fines get into the filter sock but this way you can avoid the disaster of GFO fines everywhere.
(Mr. Saltwater tank recommends the SpectraPure Dual Media Reactor (https://www.saltwateraquarium.com/mr-dual-independently-controlled-dual-chamber-media-reactor-spectrapure/) where you can adjust the flow rate through each chamber of the reactor.)
While some people test phosphate levels frequently to know when to change their GFO but I just replace it on my clients' systems at regular intervals. It is just easier that way .
I recommend using GFO from the very beginning of your system so any new corals you put into the system will never get used to elevated levels of phosphate to begin with. Elevated levels of phosphate will usually cause them to brown out because the symbiotic algae inside them overpopulate.
If you already have a phosphate issue and are introducing GFO to a well established reef, start off with a small amount and proceed slowly. Though high phosphate levels aren't good, many corals will not be happy and may even end up dying if you pull the levels down too much, too quickly.
I use and recommend GFO and if your tank already has a high phosphate issue (> 0.10 ppm), start with a little GFO and add more to slowly bring down your phosphates.