The Salty Side: Saltwater Aquariums for Beginners

What is GFO in a saltwater aquarium?

There is one constant in keeping a saltwater aquarium, the prevalence of phosphates. There are a few ways of lowering phosphate to more desirable levels, with GFO being one of them. GFO (Granular Ferric Oxide) is a dry granular red-brown media that has become a great way to reduce phosphate. In this article, we will cover GFO in a saltwater aquarium.

What does GFO do in a saltwater aquarium?

GFO is a very effective phosphate absorber. By running it either in a fluidized reactor or in a media bag, you are proactively removing phosphate from the water column. Phosphate that is constantly introduced through feeding and detritus would otherwise build up to unsuitable levels in your aquarium. This would result in unwanted algae growth and dulled coral colours. The constant absorption of phosphate by GFO helps to maintain lower phosphate levels, ideally 0.03ppm to 0.05ppm.

Do I need GFO in a saltwater aquarium?

To keep a successful, thriving reef tank, we try to mimic the conditions found in the temperate waters of coral reefs. The water found in coral reefs has only trace amounts of phosphates, which allows corals to grow abundantly and keeps algae at bay. The ocean, being an open space, is capable of maintaining those lower levels of phosphates naturally. However, our reef tanks in comparison are small, enclosed environments. They are not able to control phosphate levels naturally. In fact, without our help phosphate would continue to increase unimpeded.

GFO allows us to absorb that excessive phosphate and remove it from our tanks, bound in the GFO granules. To put it plainly, without GFO we could not run successful reef tanks. Instead, they would be overridden with Green Hair Algae, cyanobacteria and other unwanted macroflora.

How long does GFO last in a reef tank?

The longevity of GFO differs from tank to tank. It can be determined by your fish stock, the amount of food they are fed, your dedication to cleaning out your filter socks, or even how long you have had higher levels of phosphate in your tank. Assuming phosphate levels have been correctly maintained, you can expect GFO to last from 4 – 6 weeks before it needs to be replenished.

If you have not used phosphate absorber before, or you have let your phosphate get out of control, then the timeline is completely different.  When you use GFO for the first time, or to bring down higher levels, it must be understood that until the GFO was introduced, the phosphate had to go somewhere. That somewhere is normally your live rock. You may have heard the phrase ‘phosphate bound to live rock.’

To regain control of the phosphate levels in your system, you need to withdraw the rock-bound phosphate too. You may notice when you first use GFO that the phosphate level does not drop. This is because the GFO has already reached maximum absorption. You will need to replenish your GFO every 3 days for a minimum of 3 to 4 times to absorb the phosphate from your tank. Once you notice a drop in the levels when testing, you know you are close to control. Aim to keep phosphate between 0.03ppm and 0.05ppm.

What are examples of GFO for a saltwater aquarium?

There are a few different brands of GFO on the market, most of which do the same thing in the same manner. The stand out GFO’s are high-capacity types:

ROWAphos Phosphate removal media

Seachem PhosNet

Kolar Lab GFO

TL Reefs GFO Phosphate Remover

BRS also offer a high-capacity GFO with good ratings. I have always used ROWAphos for phosphate removal. I have found it to be a superb product and would not hesitate in recommending it.

For more information on ROWAphos and its use, read this article.

D-D RP-50 ROWAphos Phosphate Removal Media - 500 ml/16.9 oz
  • Best Selling Phosphate Remover
  • Used Widely all over the world
  • Largest absorption ability

How Often do you change GFO in a reef tank?

As already discussed above, this will be determined by a number of factors. If your phosphate is under control, you can expect to change it every 4 – 6 weeks.

Does GFO reduce nitrates?

GFO has no effect on nitrates. The use of GFO is normally run alongside nitrate reducing methods such as NOPOX or other methods of carbon dosing. It is also possible to achieve natural nitrate control by limiting the fish you keep and feeding them responsibly.

Does GFO lower alkalinity?

According to Randy Holmes-Farley from Reef2Reef – ‘The chemicals in GFO induce calcium to combine with carbonate to form solid calcium carbonate, which reduces alk and calcium in the water. The solid material that forms attaches to the GFO, and to surfaces just downstream of it.’

Randy goes on to say the affect is hardly noticeable.

Just starting out in the hobby?
Get your copy of How To Set Up A Successful Saltwater Aquarium ebook today.

This ebook will help to guide you past the mistakes I have made over the years.

Is GFO in a saltwater aqaurium harmful to corals?

The granules are not harmful to fish and corals, but the GFO must be rinsed with RO water or old tank water until it is clear. The dust on GFO can be an irritant to both fish gills and coral polyps.

How fast does GFO reduce phosphates?

This question has been covered above under How long does GFO last in a reef tank?

Can you rinse GFO with tap water?

Water companies actively add phosphate to water systems to prevent metal dissolution from water pipework systems. By using tap water to rinse your GFO, you will be reducing its effectiveness in the aquarium. It is best to rinse it through with RO water or old tank water.

Will GFO help with diatoms?

Most GFO’s have the ability to absorb silicates. Diatoms rely on silicates as a food source. As the silicates in the tank diminish, so will the diatoms disappear. If you have been using GFO that states it will absorb silicates, but you are still finding them in your aquarium, you have two scenarios:

  • Your RO water is not being filtered correctly. Normally the addition of or replacement of DI resin will stop silicates coming through from your tap water.

  • Those diatoms could be dinoflagellates. You will only be able to determine this accurately with a microscope.

How much GFO do I need?

The amount of GFO required will depend on the brand and instructions included on the container. It will also depend on your level of phosphate in your aquarium. The best answer to give is to make sure you purchase enough to treat your tank four times over. With sufficient GFO, you know that you will be covered should you need more than you thought.

What are the alternatives to GFO?

For more information on alternatives and general phosphate control click here, to read my article on phosphate.

Hi, my name is Craig.
I am the owner of The Salty Side.
Firstly, thank you for visiting us. If you have found this article to be informative, and you now have a better understanding of the topic, then my job is done.

If you are feeling charitable, and feel I deserve a little thank you, I thank you in advance for any support given.

Scroll to Top