What is GFO?

What is GFO?

Granular Ferric Oxide (aka GFO) or iron oxide hydroxide is a reddish brown, granular-shaped filter media that is one of the most common and effective methods of reducing phosphate in a reef aquarium. GFO media is readily available, inexpensive and lasts a long time, making it a popular choice among saltwater aquarists.

What Does GFO Remove From a Reef Tank?

In saltwater aquariums, GFO is generally regarded as a very safe and efficient way to reduce phosphates and silicates from the water. Maintaining low phosphate levels will help prevent algae outbreaks and control algae growth in reef tanks. In addition to reducing phosphate, GFO also absorbs fluoride, molybdenum, selenium, antimony, arsenic and more.

Outside the reef aquarium hobby, GFO is regularly used in municipal water facilities to safely and effectively remove arsenic. It is important to keep this in mind when purchasing GFO, as many municipalities often recycle their GFO that is loaded with arsenic, making it risky and less effective. Because of this, we do not recommend using recycled GFO for saltwater aquarium use.

How Does GFO Work?

Absorption vs. Adsorption

GFO works by the process of adsorption, by which phosphate, silicate and other chemicals adhere to the surface of the media. Adsorption is a different process from absorption, in which a substance diffuses into a liquid or solid to form a solution.

Drawbacks of Using GFO

Drawbacks of Using GFO

While GFO is generally safe to use in a reef aquarium, it is not a perfect science. And while most issues can be mitigated with proper use, here are some of the potential drawbacks of using GFO:

1. Removal of Trace Elements

Issue: GFO can remove copper, manganese and other key trace elements. In high concentrations, these elements can quickly kill invertebrates and other tank inhabitants. However, these elements are actually critical to tank health in small amounts and should not be completely eliminated from the water.

Solution: Replenish trace elements with regular water changes on a monthly or more frequent basis, or by dosing with supplements / additives that contain key trace elements.

2. Drop in Alkalinity / Drop in pH

Issue: As aquarium water comes into contact with GFO, trace amounts of precipitate bind to the surface of the GFO thereby reducing alkalinity which in turn reduces pH. This precipitate can also bind the GFO granules together reducing the effectiveness of the GFO.

Solution: Use GFO in a media reactor, as opposed to a filter media bag. The gentle churning of the media in the reactor keeps it in suspension and prevents it from binding together.

3. GFO Dust is Damaging to Fish and Corals

Issue: GFO dust can cause irritation to fish gills and coral polyps.

Solution: Carefully rinse your GFO prior to use. If using GFO in a media reactor, catch all the tank water that comes out of the reactor until the water runs clear. Replenish all the water removed with fresh saltwater. If you are performing a water change at the same time as your GFO reactor maintenance, this is easy to do. As an alternative, set up the reactor in a bucket or sink and run freshwater through it until the water runs clear.

4. Stripping Phosphate: 

Issue: Removing (i.e. stripping) phosphates too quickly from the aquarium can cause stress to corals. High levels of phosphate in a reef aquarium are no good. However, phosphate is a required nutrient for proper coral health. Therefore, dramatically changing the phosphate levels in the tank can be stressful to corals. This situation most commonly occurs when phosphate levels in the tank are very high to start and then are removed quickly from the system. 

Solution: Start with a small amount of GFO filter media and don’t use more than the manufacturer’s recommended amount.

GFO is a powerful tool in our reef keeping arsenal. Keeping phosphates at bay limits algae growth and allows corals to properly lay down fresh skeletons. While the information presented here may sound scary, it is very safe to use.  Start with small amounts, rinse carefully and change frequently. You’ll have great results!

As always, we are here to help!  Let us know if you need any assistance or have questions about your Simplicity product.

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