How Often Should I Change My GFO and What's the Most Effective Way to Use It?

How Often Should I Change My GFO & What’s the Most Effective Way to Use It?

GFO (Granular Ferric Oxide) is one of the most common ways to combat high levels of phosphates in a reef tank. Phosphates are predominantly the result of biological waste (i.e. fish poop). They are also commonly found in fish and coral foods, untreated water sources, substrates, additives and even other chemical filter media.

While coral, bacteria and other processes work to naturally combat phosphate, in most aquariums these natural processes are not enough to keep phosphate under control. So, it’s best to use supplemental GFO or other forms of phosphate removal.

The use of GFO is commonplace nowadays. However, many folks don’t use it properly, which reduces its effectiveness and, in extreme situations, can cause irreversible harm to your tank and its inhabitants.

If you are thinking about starting a GFO regiment in your aquarium, here’s what we recommend:

How Often Should I Change My GFO?

Change GFO when phosphate levels begin to rise. This is generally every 4 weeks for standard GFO and closer to 8 weeks for High Capacity GFO.

What Methods Can I Use for GFO and What are the Pros / Cons of Each?

The two most common methods for using GFO are a media reactor and filter media bag.

Media Rector (Recommended Method)

Phosban Reactor
Phosban 550
Media Reactor

A media reactor is essentially a tube that holds GFO and other forms of media, into which tank water is pumped. As water passes through the media in the reactor, phosphate and other impurities are removed.

When using a media reactor, it is very important to carefully control the speed of the water entering the reactor, so that the GFO filter media tumbles very gently. Tumbling / churning the media too aggressively will pulverize the media into dust. This will irritate corals and fish, as well as send the GFO media into the tank where it cannot be removed. On the flip side, failure to tumble the media will result in the GFO clumping and becoming less effective.

While more expensive than a filter media bag, a media reactor is significantly more effective at removing phosphates.

Filter Media Bag

Filter Media Bag
Filter Media Bag

The second option is to use GFO in a filter media bag with a micron rating of 300 or less. Filter media bags have very tiny holes in them allowing water to flow through, while containing the larger GFO particles. If you opt for this method, the filter media bag should be placed in a high flow area where the majority of water passes, like a sump.

It is also a good idea to manually tumble the media every couple of days. This is a very inexpensive method for using GFO. However it is less effective than a media reactor, because the media does not continuously tumble. This often results in clumping and less water passing through the media than with a media reactor.

How Much GFO Should I Use?

GFO is very effective at removing phosphate. However, it is possible to remove phosphate too quickly, which may stress out your corals and other tank inhabitants. In the case of GFO, more is not better. So never load up a reactor with piles of GFO thinking you won’t have to change it as often. Doing this could damage your corals’ health. As such, we recommend the following:

  1. Use small amounts of GFO and change it out more frequently.
  2. Use the same amount of GFO every time you change it out.

If you feel the need to either increase or decrease the amount of GFO you are using, make the change very gradually. The key is to keep things as consistent as possible.

So how much should you use? We generally recommend starting with ½ cup for every 50 gallons of aquarium water and no more than a ½ cup for every 25 gallons of water.  (e.g. For a 100 gallon aquarium, this would mean starting with 1 cup.)

How to Change Out Your GFO / Instructions for Use

GFO Equipment
GFO Equipment

When starting GFO for the first time or changing out your existing GFO, please follow these step by step instructions.

1. Dispose of old media (does not apply to first time use):

  • Media Reactor: Carefully clean out the media reactor. This includes thoroughly rinsing out the media reactor and any sponges used in it.
  • Filter Media Bag: Dispose of old bag and replace with a new one.

2. Add media to reactor or bag:

Carefully measure out the correct amount of media and add it to your reactor or filter media bag.

3. Rinse:

Thoroughly rinse the media to prevent the dust from reaching the inhabitants in the aquarium.

Media Reactor: Please use one of the following two methods:

Clean GFO
Clean GFO
Dirty GFO
Dirty GFO

Media Reactor Method 1:

Fill a 5 gallon bucket with fresh water and place the pump feeding your media reactor into the bottom of the bucket and the outlet of the media reactor into the sink. Pump freshwater through the reactor until the water runs clear.

Depending on the amount of GFO used, it may take a good portion of the bucket capacity to run clear. Carefully dump out any freshwater remaining inside the reactor (a touch of tap won’t hurt) prior to installing on your tank.

GFO Setup
GFO Setup

Media Reactor Method 2:

Hook up your reactor and run water through and have the outlet dump into a 5 gallon bucket or similar until the water run clears.

Depending on the amount of GFO used, this may also take a good portion of the bucket capacity to run clear. Dump out the water in the bucket and replace with fresh saltwater. Put your hand on the water as it comes out and check for grains of GFO in your palm as the water flows out.

Filter Media Bag: Simply hold your GFO under tap water until it runs clear.

4. Install

  • Media Reactor: Adjust the flow rate of the water entering the reactor so that the GFO tumbles very gently. As we previously mentioned, it is very important to have a little bit of tumbling action, however too much tumbling is never good.
  • Filter Media Bag: Simply place the filter media bag in a high flow area of your sump.

For nearly every reef aquarist, GFO has a time and a place in your arsenal of reef care. Having the knowledge to use it effectively and safely is one important step in keeping a happy and healthy reef.

Should you have further questions about Simplicity GFO, please don’t hesitate to contact us. We are available via live chat, email or phone.

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