GFO or granular ferric oxide is the most widely used filter media in reef tanks. Jen gives us some solid answers to the three most common questions around this coral-saving, algae-busting filter media. 

Why should I use GFO?

GFO is one of the two best solutions for rising phosphate levels in a reef aquarium.  Whether you're being proactive and using GFO as part of your regular filtration routine or spot-treating an unexplained spike, GFO will get the job done and help to safely remove phosphates from the aquarium water.  

Phosphate battles are extremely common for reef tank owners, especially during those first couple of years when you are working to dial in your husbandry skills and establish biodiversity in the tank. Phosphate is a necessary and natural part of your reef tank, 99% of phosphate comes from fish food and the resulting waste. Lack of proper maintenance or poor quality source water can also contribute to phosphate stacking up in your tank. Your fish and corals both require phosphate to thrive but it's all about balance. Too much phosphate can begin to poison your coral, inhibiting their calcification and growth as well as contribute to unsightly nuisance algae outbreaks. 

BRS Granular Ferric Oxide

When do I use GFO?

GFO is most commonly used in one of two scenarios, treatment or prevention. Phosphates will spike in a reef tank for a variety of reasons; overfeeding, insufficient filtration for the bioload, dead animals, poor quality RO/DI water, and even some additives. It is then important to lower those rising phosphate levels before nuisance algae begin to thrive and take a stronghold in your tank or worse yet, poison your corals. Hobbyists can use GFO to help lower phosphates down to acceptable levels along with making necessary changes to their husbandry routine to maintain low levels moving forward. 

Prevention is also very common in which we employ GFO as part of our regular filtration routine. If your tank always runs with a higher phosphate level and your filtration just cannot keep up no matter what you do, adding a GFO media reactor and maintaining it regularly will allow you to prevent those nasty algae outbreaks and reduce your worries of phosphate becoming a problem. With phosphates especially, an ounce of prevention is worth a pound of cure. On that note, stripping your tank of phosphate completely is something that can also be problematic. 

No matter your situation, the key to water quality and chemistry in a reef tank is balance and stability. Avoid the highs and lows, the extreme swings, and constant changes. Instability will stress your tank inhabitants quicker than anything. Having a phosphate test kit on hand to monitor your tanks' levels regularly is the way to go. The Hanna Phosphate Checkers are a hobby favorite for the ease of use and consistent results.

How to use BRS Bulk GFO

GFO works best inside of a fluidized media reactor which helps to increase contact time with the media granules. Using GFO passively or in a media bag tends to waste media and does not allow for sufficient contact time. There are a variety of different reactors on the market but the BRS Mini Reactor is one the most affordable and easy-to-use options that is actually large enough to handle about 90% of reef tank owners based on tank size. While you may not be running GFO 24/7, having a reactor on hand and ready to go is the best approach.

GFO is best applied as a periodic tool to maintain those unexpected spikes of phosphate. The facts are, your husbandry routine and filtration should be able to handle everyday phosphate input and GFO should only be necessary on the rare occasion you start to see levels rise. Every tank is different in this regard so we cannot give you specific instructions for how often to use it or how much GFO you're going to need.

There are two different types of GFO, the BRS bulk standard, and high-capacity which can be selected based on the severity of your problem and tank size. High-capacity being the obvious choice for those tanks requiring treatment more often or larger water volumes. Reference our GFO Instructions page for a step-by-step process and learn how to use the BRS calculator to find calculate exactly how much GFO your tank needs.