Bulk Reef Supply

BRS Bulk GFO Granular Ferric Oxide - High Capacity

Grouped product items
1/4 Gallons (1.65 lbs) Bulk GFO - High Capacity

SKU: 000621

In stock


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1/2 Gallon (3.5 lbs) Bulk GFO - High Capacity

SKU: 000622

In stock


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1 Gallon (6.5 lbs) Bulk GFO - High Capacity

SKU: 000623

In stock


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5 Gallon (35 lbs) Bulk GFO - High Capacity

SKU: 000625

In stock


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Why does BRS recommend this?

Phosphates are going to be one of the biggest battles we have with a reef aquarium.  The BRS GFO works even better in reactors due to it being denser than most other GFO.  The price is just about double of standard GFO however you only need to use half as much which equals out in the long run.  High Capacity is going to have less dust than almost all other GFO on the market and being able to buy it in bulk saves a ton of money.


High Capacity GFO is the best performing GFO we stock and capable of removing close to twice the phosphate as standard GFO by volume. We recommend this when you have a large tank or limited room in your media reactor.


Granular Ferric Oxide (GFO) removes phosphate from the aquarium and is one of the most commonly used filtration media used in the aquarium industry.  GFO is one of the few ways to easily maintain ultra low phosphate levels on a consistent basis. Maintaining these ultra low levels will help prevent algae outbreaks as well as treat existing algae issues. Your aquarium's glass will also stay clear and free of the green hue significantly longer. GFO is most commonly used in a media reactor like the BRS reactor or media bag.


Phosphate inhibits proper coral growth by incorporating itself into the corals skeletal structure which makes it difficult for the coral to grow by laying down additional calcium and carbonate (alkalinity) ions. Maintaining ultra low phosphate levels will increase the growth of any SPS or LPS coral.


Algae outbreaks are one of the most common reasons for a complete tank shut down. We recommend preventing them by maintaining an ultra-low nutrient level environment where it would be difficult for them to get out of control to begin with. It is much easier to prevent outbreaks than it is to treat existing outbreaks.


Two Types of GFO:


  • Granular GFO is varied in its shapes and requires the least amount of flow to tumble. Good for reactors.
  • High Capacity GFO is twice as dense as Granular GFO, so twice as much material will fit into a reactor. It's extremely hard and has less dust to begin with. Fewer fines will be created during use and transportation. By volume, High Capacity GFO will remove roughly twice the phosphate compared to the same volume of Granular GFO. Best overall performance.


Special note on fighting existing algae problems: Algae needs three main nutrients to grow: phosphate, nitrate and light. Reducing any one of these will significantly slow down algae growth but may not completely solve your issue. Once algae takes hold, it can be a difficult battle but it is winnable. The best offense against algae is to take preventative measures and attack nutrient before an outbreak is apparent. Use the following suggestions and be aggressive if an algae problem is already present:


  • Maintain undetectable phosphate levels with good feeding habits and use a phosphate remover like GFO. 99% of all phosphate is added to the tank via foods.
  • Control nitrate levels by reducing feedings, increasing the water change schedule and maintaining a properly sized protein skimmer.
  • Use nutrient free RO/DI water for water changes and top off water
  • Shorten your lighting period or intensity. In some cases aquarists have found replacing old bulbs that have fallen out of their intended spectrum helps as well.
  • Continuously remove as much algae as possible by hand.
  • Add predators - nothing helps an algae outbreak as much as critters who eat it all day long. Various tangs, lawn mower blennies, crabs and snails are all good options. It is also theorized that a healthy pod population will also control algae growth before it gets a chance to take root.

Note: All Bulk GFO is packaged by weight.


GFO Instructions

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Product Questions

I'm a little confused about the notion of "how much to use." Since we are not adding anything to the tank water per se, what difference does it make how much we put into the reactor? Why not just put whatever amount we like into the reactor and then monitor phosphate levels to determine when to replace? Why do we need to calculate anything? Guess I have the same question for Carbon.
Question by: John DiNovo on Jan 24, 2018, 7:02 PM
Since the goal with phosphates is to stick around the 0.02ppm mark, loading a cartridge up with GFO will actually strip out all the phosphates to the point where your corals will starve. We suggest using the proper amount to help limit the amount of phosphate removed to the amount of food and other additives that you add keeping your tank in the sweet spot. Carbon is a bit different since it is the porous structure that is trapping molecules and other unwanted toxins odors etc. Over usage of carbon necessarily cause issues with nutrient levels but it will become clogged and is not efficient in the long run. There are also other studies that suggest overuse of carbon may cause HLLE in tangs and other fish.
Answer by: Charlie on Jan 25, 2018, 4:56 PM
How long can this GFO be stored in its container, before being used in the aquarium, once opened?
Question by: Chris on Oct 30, 2017, 9:19 AM
Hey there,

As long as it is stored in a sealed container and dry, it can last pretty much indefinitely. :-)
Answer by: Randy on Nov 25, 2017, 10:39 AM
I been reading a lot about over dosing GFO and removing ALL phosphate thus starving corals. Is that an issue one should be concerned with when using this product?

Question by: Michael on Oct 9, 2017, 1:15 AM
Hey there,
Thanks for getting in touch with us in regards to having too low of phosphate to allow for healthy corals. If you are using this and are not supplementing in amino acids/ target feeding, there is a change of corals bleaching and dying off slowly due to low availability of nutrients that they need. Please let me know if there is anything else that we can assist with. Thanks!
Answer by: Tyler Schafer on Oct 14, 2017, 3:37 PM
Hello Folks, In my red Sea reefer 350, 91 gal total, I'll assume 80 gal or less total water, but based on a phone call to you folks, I purchased the 5" mini reactor and run about 1/2 cup ROX 0.8 and 1/2 High Cap GFO. My problem is, I'm wasting GFO it seems. The recommendation for the carbon is to change every 2 weeks, but the recommendation on the GFO is every 4 to 8 weeks. These don't match up well at all and I'm throwing away the High Cap GFO too early. Any better options ? I'm running one canister due to sump size limitations. Should I switch one of the two products ? thanks.
Question by: Jeff Northcutt on Jun 6, 2017, 10:14 AM
Hi Jeff,

That's a great question! What we have found to work well for us is to cut the amount of gfo down based upon your change frequency. For example, if you change your carbon every two weeks, and would normally change your gfo every 4 weeks, just use half as much gfo as the calculator recommends and this will bring the gfo capacity more in line with your carbon schedule and avoid wasting extra media.

Happy reefing!
Answer by: Zack on Jun 13, 2017, 4:02 PM
How often does this need to be replaced?
Question by: Eric on Jan 10, 2017, 8:22 AM
Great question! Typically, the standard GFO lasts me around two weeks and the High Capacity lasts me more around four weeks. Those may vary depending on the amount of phosphates in your tank and that is introduced into your tank. :-)

Answer by: Randy on Jan 10, 2017, 12:26 PM
Do you need to rinse the GFO HC?
Question by: Joshua Guzman on Mar 30, 2016, 8:09 AM
Great question!
It is a good idea to rinse GFO quickly with tank water to wash off any small dust. Feel free to let us know if you have any further questions!
Answer by: Connor on Mar 31, 2016, 6:47 PM
Hi, I have a 150gal. Freshwater African cichlid tank setup. I am interested in buying one of your GFO reactors but not sure which one for my size tank and how much HIGH CAPACITY GFO should I use in the reactor? Also will this help keep down that brown diatom growth on my rocks?? Thank you for your help!! Mark
Question by: Mark on Mar 10, 2016, 1:35 PM
Great question!
Overall you will use our aquarium calculator to find the correct amount between 2.5 cups for regular GFO and 1.25 cups for High Capacity. With this said you can use a single reactor for High Capacity and a larger reactor would be needed for the regular GFO. Feel free to let us know if you have any further questions!
Answer by: Connor on Mar 10, 2016, 4:09 PM
Will this product also remove silicates?
Question by: Ron on Jun 5, 2015, 4:34 PM
That is a great question!
GFO will help to remove silicates from the water slightly. It sole purpose however is for phosphate removal.
Answer by: Connor on Jun 8, 2015, 2:10 PM
How many cups are there in the 1/2 Gallon (3.5 lbs.) of HC GFO?
Question by: Adam on Feb 6, 2015, 5:36 PM
Great Question Adam!
There will be 8 cups in a half gallon container.
Answer by: Bulk Reef Supply on Feb 6, 2015, 5:36 PM
Can the HC GFO tumble more than the standard, higher flow? I am also using the filter pads on the top to help keep the particles in better. Your thoughts would be great.
Question by: Chad on Jul 7, 2014, 2:50 PM
Hi Chad,
The GFO HC is a denser/harder product which means it can handle more flow without breaking down (though at the same flow, its tumbling characteristics would be about the same). If you are trying to increase flow you could also remove the sponge pads in the reactor when using GFO.
Answer by: Bulk Reef Supply on Jul 7, 2014, 2:50 PM
will this product leach phos back in the tank?
Question by: Eddie on Apr 21, 2014, 5:05 PM
Hi Eddie,
It will not release back phosphates under aquarium conditions.
Answer by: Bulk Reef Supply on Apr 21, 2014, 5:05 PM
Question by: jerry on Apr 1, 2014, 2:16 AM
Answer by: Bulk Reef Supply on Apr 1, 2014, 2:16 AM
Will this work in a phosban 150?
Question by: Jeff on Nov 29, 2013, 9:32 AM
Hey Jeff,
It sure will. Its a great combo if you need something that is hang on back!
Answer by: Bulk Reef Supply on Nov 29, 2013, 9:32 AM
what is the amount of GFO HC you would normally run in a 120 gallon reef?
Question by: Skopp on Oct 15, 2013, 3:19 PM
They there,
For a tank that size you would want to run 1 cup of GFO. The easiest way to do the math is to use the BRS Calculator right here:
Answer by: Bulk Reef Supply on Oct 15, 2013, 3:19 PM
Does the high capacity GFO have to be tumbled in a reactor or could I place it in a media bag and put it in a high flow area of my sump?
Question by: Kevin on Oct 10, 2013, 10:31 AM
Hey Kevin,
It is best when tumbled in a fluidized bed reactor as the media otherwise has a tendency to harden into a brick, but even placed in a filter sock would be better then nothing.
Answer by: Bulk Reef Supply on Oct 10, 2013, 10:31 AM
How much do u start out with in your reactor? Is there a measurement per gallon ratio?
Question by: Brandon on Oct 4, 2013, 12:46 PM
Hi Brandon,
It depends on how large your tank is. The easiest way to do the math is to use our reef calculator at the link below. You enter the size of the tank and the type of GFO you want to use, it will tell you how much GFO to add.

Answer by: Bulk Reef Supply on Oct 4, 2013, 12:46 PM