Aquarium Filter Media Explained: Mechanical, Chemical, and Biological Filtration
When it comes to filtering your water, there are three areas of filtration to cover. Each of which employs different kinds of media to help get the job done. Within each of the categories, you have options for filter media that should be chosen based on what you want to accomplish.
Mechanical Filtration: Physically filtering out suspended or undissolved particulates from the aquarium water.
Chemical Filtration: Removing dissolved contaminants that changes the "chemical composition" of the water.
Biological Filtration: Harboring beneficial bacteria to biologically break down waste via the nitrogen cycle.
Filter Media is the variety of materials used to accomplish each of the above types of filtration. For example, mechanical filtration can be accomplished using coarse sponges and filter socks, chemical filtration can be accomplished using carbon and GFO, and biological filtration is accomplished with the use of bio-balls or porous ceramic media. As it pertains to aquariums, there really is a wide variety of different filter media options but they can easily be grouped by the type of filtration they accomplish.
Mechanical Filter Media
This includes filter rollers, filter pads, sponges, and filter socks. Anything that captures free-floating undissolved particles from the water. This is often one of the primary forms of waste removal in an aquarium because you can capture and remove waste and debris before it is broken down into nitrate and phosphate.
Some types of mechanical media are denser than others, making it tougher for smaller particles to go through. This density is measured in microns, where 1 micron is equal to an opening size of 1/1000 mm. A really dense felt filter measuring 50-microns will capture much finer particles compared to a more porous 200-micron filter sock. That also means the filter pad is probably going to clog up faster than the filter sock.
Tips For Mechanical Filtration Media
- Mechanical filtration is most often the first form of media to come in contact with the water as it passes through the filter.
- Depending on density/pore size, filter pads need to be changed out quite often. Usually, every 3-5 days give or take.
- If the mechanical filter media is not removed, the waste it collects is not removed from the water and bacteria has a chance to break it down.
- Coarse mechanical filter media can be reused after rinsing and drying, this includes coarse sponges and filter socks; check out How to Clean Filter Socks.
- Fine mechanical filter media is disposable, like filter floss and felt polishing pads, because they cannot be effectively rinsed clean.
Chemical Filter Media
There are a few different types of chemical filter media hobbyists can use to target various problems.
- Carbon - A broad-spectrum adsorption media that can remove a variety of chemicals and contaminants including toxins, tannins, and odor-causing compounds.
- GFO - Binds to phosphate molecules so they can be removed from the water.
- Resin Media - Used to target specific heavy metals, phosphate, and various dissolved organic waste compounds.
- Mixed (All-In-One) Filter Media - Uses a special combination of Carbon, GFO, and Resin to act as a comprehensive solution.
Carbon is one of the most widely used chemical media in aquariums because it excels at clearing yellowed water, reducing odors, and removing unwanted contaminants. It is commonly used to absorb chemical toxins released by corals, residual medications, tank treatments, and other dissolved impurities that can build up over time.
Most aquarists use activated carbon in granular form, which increases surface area and improves contact with aquarium water. Granular carbon can be placed in a media bag or run in a reactor, hang-on filter, or canister filter to ensure consistent water flow across the carbon.
Carbon becomes exhausted once its pores are filled with contaminants. In saltwater aquariums, this typically happens quickly, so carbon should be replaced every 5–7 days. While some hobbyists run carbon longer, its effectiveness usually drops significantly after the first week. Using smaller amounts and changing it frequently is more effective than running a large batch for extended periods.
Learn More: How To Use BRS Carbon Media
Tips For Using Carbon Filter Media:
- Before use, rinse the carbon with water to get rid of any carbon dust particles.
- Carbon should be used in a ratio appropriate to your tank size. Example: 1/2 cup for every 40 gallons of water volume.
- Replace carbon every 5-7 days.
- In sump use: carbon can be placed inside a filter media bag, then placed in an area of high flow where water can actively pass through it, like in between a baffle.
- Using a fluidized media reactor is the most efficient way to use carbon because it increases the contact time with water.
- Most power hang-on filters and canisters filters can accept media bags full of carbon or offer specific carbon "cartridges" for use in the filter.
Granular Ferric Oxide - GFO media is specifically used to remove phosphate from the aquarium water. The GFO granules bind to phosphate molecules in the water which can then be removed along with the media.
Alongside nitrate, phosphate is the leading cause of nuisance algae growth in aquariums and is the end byproduct of food and fish waste being broken down by bacteria in your aquarium. It is critical that our filtration systems control rising phosphate levels to keep our aquariums healthy.
While a healthy reef tank should not require to use of GFO at all times, most hobbyists keep a jar of GFO around so it can be used in the event phosphates do begin to climb. GFO should be used as a tool to fix a problem rather than a preventative measure.
Phosphate is a granular media, just like carbon, but is much denser. Using GFO in a fluidized media reactor is important to ensure you are getting the best exposure to the media. If GFO is used passively inside a media bag, the water simply flows around the media which results in poor performance.
Learn More: How To Use BRS Granular Ferric Oxide - GFO
Tips For Using Granular Ferric Oxide - GFO Filter Media:
- GFO should be rinsed before use with RO/DI water to clean off any dust.
- GFO should be used in a ratio appropriate to your tank size. Example: 1/4 cup for every 50 gallons of water volume
- In a media reactor with a low flow pump, the media can gently fluidize and tumble which allows for sufficient contact time with the media granules.
- Monitoring phosphate in the aquarium water directly is the only way to know when GFO media needs replacing.
- GFO can last anywhere from 1 - 8 weeks depending on the level of phosphate being pulled from the water.
- GFO should only be used when phosphate levels begin to rise out of range unexpectedly.
Specialty filter media make up the remaining chemical filter media which includes the various Resin Media and All-In-One Filter Media. These specialty filter media have largely been developed in the last 10-20 years and provide tank owners with alternatives and sometimes more effective approaches to chemical filtration.
There are multiple types of Resin Media used to target a variety of different contaminants including heavy metals, phosphate, and various dissolved organic compounds. The unique benefit of resin media is that they are fast-acting and, in some cases, can be recharged and used multiple times - Seachem Purigen and Blue Life OrganicRx. So while it is a bit pricier than carbon and GFO, it can be used over and over again so long as you remove and recharge the media properly.
These resins can be mixed together with Carbon and GFO to create an All-In-One Media that gives the aquarist a broad spectrum or comprehensive chemical filter media. Mixed AIO media like Chemi-Pure is rising in popularity and is especially useful in nano tanks or tanks with internal filters that just have limited space for filtration equipment. AIO media is most often used passively, with a nylon filter media bag dropped into your filtration or sump.
While these AIO media are highly effective, you can almost certainly expect the various media inside will exhaust at different rates. We know carbon exhausts quite quickly relative to resin and GFO media so the rate of use will vary drastically from tank to tank.
Tips For Using Specialty Media
- Chemical media can be used in a nylon filter media bag and placed into an area of high flow in the sump or filtration system.
- If used in a fluidized media reactor, be sure the media is packaged securely in a nylon bag and run minimal flow so as not to force the tiny resin beads out of the media bag and into your tank.
- Some ion exchange resins have the extra benefit of being rechargeable so they can be used multiple times if properly recharged.
- Some manufacturers grossly overstate how long the media will last; be sure to test your parameters and only use the media as needed to get the most bang for your buck.
Biological Filtration
Aquariums would not be safe for fish without the presence of beneficial bacteria. These bacteria naturally grow on the surfaces inside your tank which includes the sand, rocks, decor, glass walls, filtration surfaces, and biological filter media. The beneficial bacteria is what enables the nitrogen cycle and is the means by which waste and leftover fish food is broken down. Without the bacteria, fish would immediately be poisoned by the presence of ammonia in the water.
In an aquarium, hobbyists can supply special biological filter media (bio-media) which simply provides extra surface area for the beneficial bacteria to grow. This includes ceramic rings, bio-blocks, bio-balls, and a variety of other inert porous materials. The bio-media is placed into an area of the filtration where water can flow through it and, once established with bacteria, helps the aquarium remain biologically stable. Bio-media does not need to be replaced and will remain effective as long as it stays wet and the bacteria is alive.
Tips For Using Bio-Media
- Don't allow bio-media to become clogged with detritus. Rinse regularly in salt water to wash away detritus.
- Place into an area where water is forced to flow through it, not just around it.
- Don't let bio-media dry out once it has become established with bacteria, this will kill the bacteria.
- Bio-media takes 2-4 weeks or more to cycle and become populated with beneficial bacteria.
Biopellets are a little different from your classic Bio-Media but still work by harboring beneficial bacteria. You might think of them as super-charged bio-media that helps remove nitrate directly as opposed to helping create it.
Biopellets are plastic beads composed of biodegradable polymers that provide a surface for bacteria to grow while also supplying a source of organic carbon to fuel the bacteria. The surfaces of the beads become coated in a biofilm full of bacteria that have used up available nitrate and phosphate from your aquarium water. The nitrate and phosphate-laden biofilm is then tumbled off the media and exported from your aquarium via a protein skimmer.
Biopellets must be used inside a special biopellet reactor with substantial water flow to keep the pellets tumbling. A reliable protein skimmer is also recommended to help export the biofilm.
Learn More: How To Use BRS Biopellets
Tips For Using Biopellets:
- Biopellets must be used in a biopellet reactor with enough water flow to gently tumble the pellets.
- Prior to use, biopellets should be soaked in aquarium water over 24 hours or longer until they're no longer floating.
- The outlet of the biopellet reactor should be directed towards an efficient protein skimmer, which will cause more skimmate production.
- When using biopellets, phosphate control media such as GFO should be suspended or drastically reduced.
- It is best to start with 25% of the recommended amount of biopellets for your tank size, then gradually increase as needed to maintain ideal nitrate levels.
Filter Media Q&A
Filter media can feel complicated fast, but it really boils down to three jobs: mechanical, chemical, and biological filtration. Use the Q&A below to pick the right media (and use it the right way) without turning your sump into a science fair.
What are the three main types of filtration, and what does each one do?
- Mechanical: physically traps undissolved particles like detritus and food.
- Chemical: removes dissolved contaminants by adsorption or binding (think carbon and phosphate removers).
- Biological: provides surface area for beneficial bacteria that drive the nitrogen cycle.
What kind of filter media do I need for a saltwater reef tank?
Why is mechanical filtration usually the first stage?
What does “micron size” mean for filter socks and pads?
How often should mechanical filter media be changed or cleaned?
What does activated carbon do, and how should it be used?
- Rinse before use to remove dust.
- Run it in a media bag in high flow, or in a reactor for best efficiency.
- In many saltwater setups, carbon is most effective when replaced every 5 to 7 days.
What is GFO and when should it be used?
- Rinse with RO/DI water before use.
- It performs best in a media reactor with gentle tumbling for contact time.
- Service life can vary widely (about 1 to 8 weeks), so testing phosphate is the real indicator for replacement.
What are resin media, and what makes them different?
What is All-In-One (AIO) media and who is it best for?
What is biomedia, and do you need to replace it?
How long does biomedia take to “cycle” and start working?
What are biopellets and how are they different from standard biomedia?
What is the biggest “rookie mistake” with filter media?
With over 25 years of experience keeping aquariums of all kinds, Robert has a detailed understanding of advanced aquarium techniques and theory. Working as a professional content creator in the saltwater aquarium industry for over 18 years, his perspective, thoughtfulness for the hobbyist, and a general understanding of the hobby, and industry that surrounds it, allows Robert to write and create digestible content that makes reefing fun and easy.
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