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What is GFO and Why Is It Useful to Your Reef Tank?

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GFO  — Granular Ferric Oxide  — is a variation of iron injection used to adjust the phosphates level in the tank. Iron plays a crucial role in the formation of plant tissues and ensuring their respiration. The purpose of this article is to help you understand the importance of iron and GFO concepts.

Here is what you’re about to discover:

  • What is GFO?
  • Why is GFO Important?
  • When Should You Use GFO?
  • GFO application instructions

What is GFO?

GFO is a specific form of iron: It reacts chemically in contact with phosphates and some other substances. The resulting water-insoluble substances are then removed from the aquarium by filtration.

Granular Ferric Oxide is one of the variations of Iron Oxide Hydroxide. Granular is the form of substance: small and compact particle; Ferric is the chemical state (+3) of iron; Oxide is a chemical term for binary compounds with oxygen. 

People often confuse Ferric with Ferrous. Both terms refer to iron. Ferric is iron in +3 state while ferrous is +2. GFO is always ferric, the most stable iron compound within aerated environments (contain air).

Tip: Iron = Fe (Ferrum)

The difference between the +2 and +3 states is the resulting reaction. These substances react differently with phosphates. GFO uses Fe+3 in the form of an oxide, which binds phosphates, resulting in a precipitate. If you add iron directly to the aquarium, Fe will react with the water to produce Fe(OH)3, which is insoluble in water. As a result, the water in the aquarium will become cloudy and dirty. To prevent this from happening, use a dehydrated version of Fe(OH)3  — oxide. GFO is a form of iron that enters the aquarium bypassing the instant reaction with water but reacts well with phosphates. 

Why is GFO Important?

GFO is a popular form of chemical filtration. Even though GFO is a relatively new filtration media to help control phosphate, it’s one of the most effective methods.

Managing phosphate levels helps the corals in their struggle with algae outbreaks. Additionally, phosphates directly inhibit stony corals’ growth. A high enough concentration of phosphates can seriously interfere with the corals’ ability to build a new skeleton. This is obviously bad because it makes coral vulnerable and causes stress. 

GFO helps binding phosphates chemically and thus effectively neutralizing them. Unlike other chemical filtration media (like activated carbon), there is such a thing as too much of a good thing when it comes to GFO. Even though ferric oxide is not an ultra-high-tech missile only targeting phosphates, it does the job. However, GFO can lower trace elements that contribute to stable water chemistry. That is why you should use ferric oxide wisely: only when it’s needed; and only recommended concentrations. 

Hint: Check out this scientific research about algae effect on corals


If you ever happen to overdose GFO in your display tank, your corals might like they’re a little burned out. In other words, they’ll start bleaching. To fix that, remove GFO immediately and perform partial water changes until you see a noticeable difference.  

Here are some preventative tips:

  • Partial aquarium water changes should be carried out if the nitrate level is above 20 mg/l. Nitrate levels above 40 mg/l are considered critical and require an emergency water change + GFO (or any other media filtering)
  • Creating a denitrification zone in the aquarium can extend non-GFO treatment
  • Observance of sanitary regulations. Especially for fish: one 3-4inch-sized fish per 10 liters of aquarium volume. Moderate feeding.
  • Clean your gravel, soil, etc. Consider siphon vacuum to wash off dead animals, wastes, uneaten food, any leftovers. Schedule cleaning once or twice a month. 

Fight against algae is a tough war (especially filamentous)   — a very long and difficult one. Divide et impera: divide and rule instead of open field battle. In our case, specifically, prevent an outbreak and rule your aquarium happily ever after.

When Should I Use GFO? 

GFO is widely used as a filter media. Generally, you should leverage GFO to absorb phosphates in your tank. There are several reasons to keep phosphates low. One of the biggest  — algae outbreak.

High phosphate levels contribute to some nuisances of algae growth in the display tank. Of course, algae brings a whole range of problems. Every fish, coral, etc compete to conquer as much space as possible. Now imagine algae spreading all over the tank. They do the job much better than most organisms within the tank. Shortly, corals will be totally obliterated.

To detect high phosphates, consider getting a special test kit. They’ll reveal elevated phosphate levels. When measuring, the readings should be close to undetectable. If that’s not the case, consider partial water changes as the first step. Observe the situations for a couple of weeks (approximately 3 to 5 water changes) before utilizing GFO.  

Excessive application of GFO can lead to reduced phosphorus levels. This is especially bad for saltwater aquariums with green plants. In case of a phosphorus deficiency, the leaves will turn redder, finer, and narrower. Corals also react badly to high doses or prolonged exposure to GFO. The aggressive effects of ferric oxide show up in corals’ retractions, infections, bleaching, etc.

GFO application instructions

Once you’ve decided to use GFO, you should do it safely. You’ll find general recommendations about dosage and time frame below. If you’re already familiar with the best dosage, skim to the part about methods of application.

Tip: 1 ounce = 28g ; 1 tablespoon = 10.5g

General Recommendations (Dosage & Time Frame)

You can get either Granular GFO or Highly-concentrated GFO. Each has some features when it comes to the application. 

Granular GFO

  • For 4 gallons of tank water apply 1 tablespoon of powder. A full cup contains around 16 tablespoons.
  • Follow the above dosage for at least 4 weeks, the optimal time frame is 8 weeks.
  • Starting from the 9th week you can safely increase the dosage. Consider 2 tablespoons of GFO per 4 gallons of tank water.


As the name states, this GFO has a higher concentration. This means you should apply it wisely:

  • Consider 1 tablespoon per 8 gallons of tank water for a smooth start.
  • The period between the 4th and 8th week is your safety window. Once passed, you can increase the dosage twice. The maximum recommended HC GFO is 2 tablespoons per 8 gallons of tank water.
  • Ideally, you may want to change the type of GFO when phosphates are stabilized. It usually happens between the 4th and 8th week.

Higher concentrations of GFO are useful for a long-term approach. However, if you’re planning on HC GFO, make sure to check the dosage twice. Don’t forget about the tank’s volume. 

Methods of Application

There are two basic ways to apply GFO. Through a filter bag or using a media reactor.

How to Use In a Filter Bag: 

  1. Drain the GFO into a filter bag. Make sure it’s closed firm
  2. Use reverse osmosis water to wash off. Alternatively, place it under a faucet. Make sure water is transparent
  3. Consider placing GFO in an area with high water flow. Both in the aquarium itself or the sump. More water passes through the GFO  — the better
  4. Change the phosphate removal media every 6-8 weeks.

The best way to implement GFO is through a media reactor. GFO works best when slightly tumbled. A reactor is a simple device that sends water through the media by way of the diffuser plate at the bottom. The water passes through this diffuser plate helps keep the media in a light suspension. Also, it prevents clumping. Usually, reactors have a sponge at the top unit. It prevents any fine dust from re-entering your aquarium. 

How to Use Through the Media Reactor:

  1. Utilize an appropriate, well-working reactor to place GFO
  2. Make sure the reactor’s return line is put into a big enough container
  3. Switch on the supply pump to wash off the debris from the GFO. Wait until the water is see-through 
  4. Put the return line into the aquarium
  5. Minimize the water flow until GFO floats
  6. Control the tumbling. GFO should be barely visible on the surface
  7. Once phosphates rose, change the media. Usually, it takes around 6 to 8 weeks.

You should be fine following the instructions. Check these extra tips as well:

  • To get the best results and to increase the adsorption efficiency, place the material between two layers of filter wool or use a multi reactor (fluidized bed filter)
  • Make all changes in the aquarium carefully and slowly. 
  • Check phosphate concentrations with a test kit frequently.
  • DO NOT apply if your aquarium already has low phosphate concentrations.
  • The filler may become warm the first time it gets wet. This is normal.
  • The red staining of the water is caused by fine dust attached to the adsorber. This is not harmful and disappears quickly
  • Store GFO in a dark and cool place. 

If you’re having an algae invasion, GFO is a good “patch”. But don’t forget that this method won’t heal the wound itself: you should fix the reason behind an outbreak. Consider additional methods. For instance, water changes, adjusting lights, regulating the amount of food within the tank, adding some algae-eaters (snails, hermits). Use several methods combined to fix the algae problem.