The three foundations of filtration are biological, mechanical, and chemical filtration, each of which plays a unique role in filtering water making it safe for your aquarium pets. We are going to focus on chemical filtration and by definition, chemical filtration is anything that changes the chemical composition of the water. When it comes to aquariums, this is most often accomplished via chemical filter media such as carbon and GFO.

What Is Chemical Filtration?

As we stated above, during chemical filtration the chemical composition of the water is changed. The reason we use chemical filtration is to remove contaminants from the water which is accomplished using specialized chemical filter media.

You can think of chemical filter media like selective sponges, they absorb things but only absorb what they want and leave the rest.  Typically those things are harmful or unsightly contaminants such as heavy metals, dissolved organic and inorganic compounds like phosphate or nitrate, compounds that create odors, and can even help with clarity by removing tannins (things that make the water yellow).

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Types Of Chemical Filtration

The two most common chemical filter media used in saltwater aquariums are Activated Carbon and GFO - Granular Ferric Oxide

While the focus of our content here is around activated carbon and GFO, there really is a wide variety of different chemical filter media available to aquarists. There are various media made from special polymers or resins, like Seachem Purigen and Blue Life OragnicFX. There is also mixed all-in-one media like the Chemi-Pure lineup or Brightwell Aquatics Purit that takes advantage of carbon, GFO, and resins mixed together. 

You can find some awesome videos on BRStv where Ryan and the team dive deep into the science behind carbon and GFO. Check it out! 

Activated Carbon

Good ol' activated carbon, the media wonder.  Carbon is an excellent and very popular filter media because it is inexpensive and extremely powerful in terms of what it can remove from your aquarium water.  

Things Carbon Will Remove:

  • Chlorine and Chloramines
  • Certain DOCs-dissolved organic compounds
  • Odor causing compounds
  • Tannins or stuff that makes your water yellow
  • Most Medications
  • Contaminants or harmful chemicals

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GFO - Granular Ferric Oxide

GFO is designed strictly for removing phosphate and some level of silicate from the aquarium water. Phosphate is a critical component of your aquarium ecosystem and must be balanced correctly in order for the biological foundation to thrive. We start to see problems such as algae and various other pests as well as bacterial instability when phosphates are not kept in check.

Running with little to no phosphate can be just as risky as running with elevated levels of phosphate.  

The thing to take away here is you have to run your GFO correctly for good results. Your better off not running any GFO and just doing more water changes should you find yourself getting impatient or not capable of monitoring phosphate on a granular level.

Between the two, Carbon and GFO, 99% of our chemical filtration needs will always be met. Combine that with effective mechanical and biological filtration and you have the filtration trifecta for a happy and healthy aquarium.  

Why Is Chemical Filter Media Important?

Chemical filtration is like a tool in your aquarium toolbox, you can use it when you need it. Some tanks won't require constant chemical filtration use while others will benefit from chemical filtration running 24/7. It's all based on your particular tank and water quality.

For example, some water municipal water supplies are loaded with chlorine or chloramines, carbon is the answer.  Sometimes phosphate and even nitrate can be present in the source water too.  Some tanks run with a heavier bioload and will need a constant presence of GFO to keep that phosphate in check. Over time as organic compounds build in your aquarium, the water will begin to turn yellow and carbon will clear that up.  If your fish fall ill and you decide to use medications, carbon will remove it.

Do I Need Chemical Filtration?

No, but it really just depends on your tank. We find it good practice to always have a jug of activated carbon on hand with an appropriate media bag just in case of emergencies. Carbon can really help you out in a pinch should something contaminate your aquarium or it starts to smell. Most aquarists just find it useful. GFO is also pretty common, especially among reef tank owners where you have to be persnickety when it comes to monitoring and controlling phosphate levels. 

That said, when first starting out it is likely you will NOT need to run chemical filtration and if anything, just a little carbon. Furthermore, if you use chemical filter media properly you will wind up causing more harm than good. Facts are, modern filtration equipment and techniques are extremely powerful.  A well-built filtration system combined with responsible stocking and husbandry techniques will often negate the need for constant chemical filtration. It really is only in the event something goes wrong or goes undetected by you, the aquarium owner, will you find yourself needing chemical filtration.  All that being said, just about every tank will benefit from chemical filtration at some point during its existence, just not all the time. 

For example, phosphates can build up easily with overfeeding or the use of new food, nitrates stack up when you neglect to change your water, the tank can start to smell if your protein skimmer goes on the frits, even air fresheners and scented candles can contaminate the tank through aerosol transmission. It is just part of the game. 

How To Use Chemical Filter Media In An Aquarium

Chemical media is specifically used in two different ways, you might call it passively or actively, but to use terms that make more sense, we either use a media bag or media reactor. A media reactor (active) is always going to be more efficient/effective than a filter media bag (passive) because it maximizes water contact time with the media. 

Carbon and AIO mixed media are typically safe for use either way. GFO on the other hand, you're better off just using a media reactor, it's considerably more effective and will reduce media waste.  


  1. Measure only the appropriate amount for your water volume. NO MORE and NO LESS
  2. Pour the media in a filter media bag.
  3. Rinse with RO water or place under a faucet until the water runs clear.
  4. Place the media bag in a high flow area of the tank or sump to maximize water flow through the media.
  5. Change out media as needed.


  1. Measure only the appropriate amount of media for your water volume. NO MORE and NO LESS
  2. Place the media in a suitable media reactor.
  3. Place the reactor's return line into a bucket or sink.
  4. Turn on the feed pump to flush the media free of fines/dust until the water runs clear.
  5. Install the media reactor on your tank and place the return line appropriately into your sump or filtration.
  6. Reduce the flow through the reactor to accommodate the needs of the particular media you are using.  
  7. Change out media as needed.

To learn more, we have some great instructional guides for both activated carbon and GFO. You're also welcome to contact our friendly, talented, and highly-skilled Customer Service team anytime for one-on-one support. 

Can I Use Too Much Chemical Filtration?

Yes, this is most definitely possible, specifically with GFO.  While carbon is pretty safe for use whenever you feel like you need it, GFO needs to be monitored. Stripping your tank completely free of phosphate is dangerous territory and something ALL new saltwater tank owners do not want to flirt with.  It can lead to instability, promote pests and even crash the biological foundation in your aquarium.

You also need to be careful how quickly you reduce phosphates.  In a scenario in which you find your phosphate levels are elevated, dropping them too quickly will stress out corals quicker than anything. The idea is to only use the right amount for your tank size and TEST YOUR PHOSPHATE LEVELS on a weekly basis so you can monitor the drop and determine when the media becomes exhausted or is at maximum capacity.

This brings up a good point, chemical filter media has limits. All chemical media will become exhausted, at some point, it no longer works and needs to be removed.  Leaving chemical media in your tank longer than it should is a terrible practice. Carbon in particular exhausts quickly, within 7 days in most cases. GFO on the other hand will last 2-6 weeks in many circumstances but that all depends on the amount of phosphate you're dealing with. The general rule, replace carbon weekly and replace GFO when you notice phosphate levels begin to rise on your weekly tests.