aquarium reactor media

Aquarium reactor media

There are three popular types of aquarium reactor media:

What is activated carbon?

Activated carbon is an old-school filtration media that’s been around since the dinosaurs.  It is one of the purifying agents inside a Brita brand water filter if you are familiar with those. Activated carbon is used because it is great at removing chemicals from water. Activated carbon will remove:

  • chlorine
  • heavy metals
  • dissolved organics
  • pesticides (hopefully you don’t have this in your water)
  • tannins
  • dyes
  • phenols
  • odors
  • ozone

With activated carbon, two things are important—the size of the particles and the shape of the particles. Round, regularly shaped, smaller particles will pack better and provide you with more surface area, which gives you more filtration capacity. Larger, irregularly shaped particles give you less surface area and also will pack less closely, which will give you less overall surface area and will have a more limited filtration capacity. Activated carbon is one of the most popular and inexpensive aquarium reactor media options.

Caution: A link between activated carbon and HLLE

I do advise that you proceed with caution when using activated carbon. There was an article published in 2011 that indicated that carbon is a causative agent in HLLE disease. HLLE stands for Head and Lateral Line Erosion, a condition where a stressed fish develops sores on the head and lateral line.
According to the study, there were carbon dust particles that were found embedded in the lateral lines of some of the fish who were suffering from HLLE.
If you plan to run activated carbon in your tank, I encourage you to read these articles. The short version of my take away here is that it is absolutely essential that you thoroughly wash the carbon to remove any fine (dust) particles. Don’t add the carbon to your water until you are comfortable that it is free from fine particles. Also, use an abundance of caution (ie. avoid it) if you are keeping fish that are highly susceptible to HLLE, like certain tangs.

What is Granulated Ferric Oxide (GFO)?

Granulated ferric oxide is an aquarium reactor media that is great at removing phosphates and silicates from your aquarium water, two water pollutants that are exceptionally “helpful” in growing problem algae in your tank. The name granulated ferric oxide sounds scary, and more like something we would try desperately to remove from our tanks, rather than put into them to purify the water, but GFO has been safely used in aquariums to purify the water for a long time.  If you don’t like the term granulated ferric oxide, you could just call it iron rust…but that doesn’t give me any more comfort when adding it to my tank, so I will just stick with calling it GFO. It sounds more mysterious that way.

How does GFO work?

GFO granules have a high surface area–which is a pre-requisite for any filtration or reactor media. Reactions typically occur on the surface, so more surface area means more room for reactions, which in turn means more capacity for filtration.

Think of GFO as a magnet for phosphates and silicates and some other metals. When the metals are forced across the surface of the GFO by the water current, the ions are drawn in by the ionic charge on the GFO (not exactly magnetism, but a similar concept) and they stick to the GFO, which essentially pulls them out of the water and removes the threat they pose to your saltwater aquarium.

Eventually, you will use up all the available spaces on the surface of the GFO–at which point you will need to replace it with fresh media.

I’ve never heard of GFO before, is it sold under other GFO trade names?

Granulated ferric oxide is the name of the chemical. It is a generic chemistry term that would be understood by any chemist or chemical manufacturer. In other words, granulated ferric oxide is a commodity. Commodities don’t command premium prices in the market, premium brands command premium prices, so granulated ferric oxide (GFO) is sold under a few popular trade names you may have seen before:

  • Phosban
  • PhosphateOut
  • RowaPhos
  • Phospure


Some of the phosphate-removing products are marketed under the chemical name Granulated Ferric Hydroxide…which is essentially the same thing as GFO.

Phosguard is NOT GFO. Phosguard is a different product altogether called aluminum oxide, but it also can be used in a similar fashion as a filtration and/or reactor media.

What are Biopellets?

I will attempt to define what biopellets are by describing what they are not: Activated carbon and GFO are aquarium reactor media that work via a chemical reaction. Ions in the water react with ions on the surface of the media and become chemically bound together and removed from the water column. Biopellets are an aquarium reactor media that operate on a completely different system. They don’t react with the water at all (in a meaningful way to the topic at hand), but rather, act as food, for beneficial bacteria that will consume pollutants in your water…pretty cool, huh?

Carbon dosing: Solid dosing vs. Vodka dosing

Another term you may have heard about is carbon dosing. Essentially, this is when you purposefully add a dose of biologically available carbon to the water, to act as food for the bacteria that will then remove pollutants like nitrates and phosphates from your water.

When some people talk about carbon dosing, they are actually adding vodka (or another distilled spirit) into their tanks, undoubtedly a good time for the bacteria involved.

Other times, carbon dosing refers to the solid dosing, which happens by way of adding a biopellet media to your sump or a reactor.

Aquarium Reactors

One of the most efficient ways to use aquarium reactor media is to add an aquarium reactor to your system.

Aquarium reactors are the subject of another article you can find here, but the premise is that reactors are designed to optimize the contact time between the water and the surface area of the reactor media. That way they take full advantage of the (nearly) entire capacity of the media.

A lower-tech option

If you don’t currently use an aquarium reactor and want to test out the benefits of using reactor media in your tank prior to making the commitment to buy and in star a reactor, you can simply buy the media and a media bag and put the media bag in an area of high flow in your sump.

The media will not work as efficiently as it would in a reactor, because it will lie flat on the bottom of the sump (or wherever you put it), but it will work, and any surface area within the media bag that is exposed to a reasonable amount of flow will do its job.

How to decide what reactor media you need?

In some ways, it would be easier if there was just one type of reactor media, wouldn’t it? There would be no decision to make. Here are some thoughts to help you decide what type of reactor media is right for your tank.

Do you have to pick one?

The first thing to consider is whether or not you really need to make a decision in the first place. A lot of people will plumb reactors in a series so that they essentially purify the water from more than one approach. If this is a possibility for you, in terms of monetary, time and space investment, you may want to think about it.

Which water parameters do you need to improve most?

A practical consideration for deciding what reactor media to use is to think about which water parameters you need to improve most.

  • Nitrates – you want biopellets
  • Phosphates or silicates – you want GFO
  • Odors, other chemicals – you want activated carbon

A few words of caution

I could probably add these same few words of caution to every post, but I do think the context here warrants some additional caution. You need to take things slowly in this hobby, under most circumstances. You can’t ignore or delay handling massive problems, but little problems are generally best dealt with by deliberate, cautious improvements, rather than dramatic changes. The aquarium reactor media we have access to, today, are powerful media, capable of significant improvements in water quality. If you have had your tank running for 5 years one way, you don’t want to all the sudden blast your tank with 3 over-sized reactors overnight. I recommend you start out small and increase over time.

The other thing not to ignore is the link between activated carbon and HLLE, a very serious condition. You should pay close attention when adding carbon, especially if you keep susceptible species and discontinue the use of carbon if you see any worsening (and scale up your particle filtration capabilities in the meantime).

aquarium reactor media

Your experiences with aquarium reactor media

What has your experience with reactor media been? Any thoughts, advice or real-world evidence to share?

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