Filter Media Types Guide

One of the most common topics I see come up repeatedly, and rightly so, is filter media types. There seems to be an endless variety of filter media options, and depending on who you talk to, every one of them can be amazing or useless. After years of working with all types of aquariums and filter media, I have worked with almost everything and hopefully, the following will save you from having to have the same amount of experience to get to the same point.

Media Order

best way to setup a canister filter

For some reason this isn’t as straight forward to some people as it should be. Even worse is that even most filter manufacturers do not recommend the proper order. The water should go through the filter media in the following order: coarse mechanical, fine mechanical, basic chemical (carbon), specific chemical, biological.

In filters that use both coarse mechanical and fine mechanical medias the coarse should always be first. Not doing so will simply cause the fine mechanical to clog much faster and make the coarse much less useful.

If you are using multiple types of chemical filtration, the water should go through the basic chemical media first (carbon) and then more specific chemical medias. In general, this won’t apply except in cases where you use both carbon and something such as Purigen, GFO, or other specific chemical medias. This is so that the carbon can remove everything it can leaving only the stuff the specific media needs to focus on so that it is only used up on what it is made for.

Finally, the water should go through the biological media last so that the bacteria are dealing with the cleanest water possible.

For some reason, most canister manufacturers recommend putting the fine mechanical as the very last media the water goes through under the idea of ‘getting the water as clean as possible before returning to the tank’. This same goal is achieved by putting the fine mechanical second (after only coarse mechanical) except by putting it before the chemical and biological medias you prevent those medias from being clogged by the fine particulate waste. This waste can completely clog the biomedia, undoing the massive benefit of all the pores and channels that house bacteria. There is absolutely no benefit to putting the fine mechanical media after the chemical and biomedia, but there is a significant downside.

You do not need to use all of these types of filter medias. You can have as little as coarse mechanical and biological. This guide is meant to cover both the basics and the most advanced needs you may have. The chart above shows the order the media needs to be in IF you need to use it all.

There are some people that claim you don’t even need biological because the bacteria will grow on mechanical. While this is true, I do not recommend this because as the mechanical traps more and more debris, the bacteria will be smothered and forced to grow on the debris, not on the media. When you go in and remove that debris, you risk removing way too much good bacteria along with it. Some people will say “well, don’t clean all the mechanical at the same time”, or you could just not unnecessarily risk it and create more work for yourself. Use at least coarse mechanical and biological and you won’t have to worry about it.

Filter/Media Bags

Many of the medias listed below will need to be contained in filter bags in most filters. They are too fine to simply be tossed into a canister’s trays (depending on the media and canister). In these cases, you want to use a good filter bag. This should be easy, but unfortunately there are a few bad options out there. I prefer Acurel Filter Bags. They are durable, their drawstring is thick making it very easy to tie and untie over and over again (unlike some that literally use thread), and they can be used over and over for years.

Mechanical medias are probably the simplest. There are few variations and every filter out there has some. Mechanical media simply collects debris and particulate waste. The mechanical media collects all that waste, it is your job as the aquarist to actually remove it from the system. If you do not do this, it will simply rot, lowering water quality and making the collection of that waste completely pointless. All filters need to be cleaned at least once per month. Depending on the type of mechanical media you have, it will either need to be cleaned or replaced during that monthly cleaning.

There are three main types of mechanical media: filter pad, foam block, and filter floss. Filter pads are what is usually built-in to those dinky slide-in cartridges that most hang-on-back filters (HOBs) have. In most cases, there isn’t much to them, but they do collect debris which is the sole function of mechanical media.

Foam blocks are common in AquaClear HOB filters and almost all canister filters. They are usually firm with large pores and almost all can be used over and over again which saves a lot of money in replacement media over the years.

Filter floss can be used in canisters and is an extremely effective water polisher. You can buy aquarium filter floss or 100% polyester pillow stuffing at a crafts store (which is much cheaper), just make sure there is no mildew protection or anything like that, just 100% polyester. In something like a Fluval or other canister with deep trays, you can pack filter floss in pretty tightly. The tighter you pack it in, the more effective it is.

Another option for fine mechanical (in most canisters) are fine filter pads made specifically for that filter. Although filter floss is best, some canisters with shallow trays simply won’t allow you to pack it in the way it needs to be in order to work best. Most canisters have an optional fine or even polishing filter pad. Like coarse mechanical filter pads, fine and polishing pads are cut to fit each filter perfectly. So if you have a canister with trays too shallow to properly use filter floss, the manufacturer’s fine or polishing pad may be the best option. The only downside is the fine pads are usually much harder to clean effectively and reuse, meaning you won’t be able to reuse them more than once or twice, if at all.

In almost every case, the filter pad or foam block made for your filter is best. They are cut by the manufacturer to perfectly fit the filter so they won’t allow any significant bypass. The other options are usually a lot of work to get to roughly the same point. Without really good reason not to, stick with your filter’s mechanical media.

Chemical Medias

There are many types of chemical medias out there. Most common, and usually included in all filters, is carbon. Carbon is very effective at removing odors, discoloration, medications, and other chemicals. In most cases, it can be better to use it than to not use it, but it does remove good things too such as certain trace elements and micronutrients. If these are the limiting factors in your tanks (rare, but it does happen in very well-balanced planted and reef tanks) then the use of carbon can have a visible impact on the growth and condition of the plants or corals. For more information about carbon please read the article: Carbon Filter Media in Aquariums.

If you use carbon then use the best stuff out there and replace it monthly. I prefer Seachem Matrix Carbon. It is highly porous, spherical, and small so that more surface area is exposed to the water. The best second option is Acurel’s Extreme Activated Carbon Pellets. This is larger but still a very high-quality carbon, and some of their larger containers are very cost-effective per weight. As a third option, any other pelletized carbon will work well. Always rinse thoroughly before use. Be aware that odors and discoloration are also, and better, removed by water changes. Odors and discoloration usually indicate that the water changes are inadequate. You can completely replace the need for carbon in any tank with adequate water changes.

Other chemical media types are things such as Seachem Purigen, Poly-Filter, granular ferric oxide (GFO), crushed coral, peat, peat granules, ammonia chips, nitrate absorbing media, and more.

Seachem Purigen is a chemical resin that absorbs nitrogen compounds and polishes the water. Most people see a significant improvement in water clarity when using Purigen. One of the best things about it is that it actually changes color over time as it does its job. It starts out white and becomes tan and then dark brown. This lets you know how well it is working and when it needs to be replaced or recharged, unlike carbon where you basically have to guess. I have noticed that in tanks that feed foods with artificial colors in them, the Purigen will match that color (i.e. artificially red foods turn it red). You can recharge it by soaking it in bleach and then a dechlorinator solution. Although I don’t like to rely on products like this for my own tanks, I like to just do a water change which does even more good, it can be very effective and does make a big difference on clients’ tanks who will not pay me to do water changes often enough and don’t do them on their own. This isn’t just for neglected tanks though, it is good for any tank especially something like a planted or reef tank. The 100mL comes in its own micromesh bag (this stuff is super fine) and can be tossed into any filter (HOB, canister, etc.). The other sizes need to be used in either a media reactor (a great addition to your filtration on a reef tank) or in Seachem’s “The Bag” which is specifically made for Purigen. I find it easiest to use the 100mL, or a reactor if I will be using enough.

Poly-Filter by Poly-Bio-Marine is another great addition to many tanks. It is a pad with a special coating that absorbs all sorts of bad stuff. Like Purigen, it starts off white and changes color, turning a very dark brown as it becomes exhausted. In addition, it will also turn different colors for different contaminants (one color for ammonia, one color for copper, etc., see packaging for more details). Also like Purigen, it is great for tanks that are a little neglected, but can still be a great addition to any tank. I usually use both Poly-Filter and Purigen on clients’ tanks.

Granular Ferric Oxide (GFO) looks like carbon except it is brick red. It is used for removing phosphates and silicates from reef tanks. Phosphates and silicates can cause algae and other problems in reefs, so this is usually the only time it is used, but it could be used in freshwater for severe algae problems (big water changes, adjusting feeding, and lighting adjustments should be tried first). It is best used in a media reactor with a flow that produces a slight tumble. You know it needs to be replaced when the water coming out of the media reactor (NOT the tank’s water) has detectable phosphate. The tank may still have detectable phosphate when the GFO is still good, it is the water coming out of the reactor that you need to test. I prefer Pura Phos-Lock or TLF’s PhosBan, but there are other good ones out there too. The only one I would avoid is Brightwell Aquatics’ GFO, it is exceptionally dirty. I have gone through over 5 gallons of RO water and still had it coming out dirty, so pass on that one. Make sure you rinse it well with RO water before using it. if you are using it in freshwater and don’t have RO, it’s okay to do a quick rinse in tap water. If you are using it in something other than a media reactor (canisters and HOBs are fine), use a good filter bag.

Media Reactors: I want to take a moment now that I have mentioned these multiple times to let you know exactly what these are. If you have never seen one before, a media reactor will look like an odd canister filter. Water goes in, goes through the media, and comes out. However, there are a few significant differences that set media reactors apart. Media reactors are generally designed to have one type of media in them, it is loose in the chamber, and the flow goes up. This creates a tumbling effect to the media. This is dependent on flow though. Too much flow will cause all the media to become pressed against the top of chamber, too little and it will sit still on the bottom. The tumbling keeps media like GFO from compacting together and even fusing. It also makes sure all the media is exposed to the water and does its job. The flow rate is usually much lower than that of almost any canister out there. With most media reactors, you buy the pump separately. I prefer the small Rio pumps/powerheads. Rios are great because they have a built-in strainer, they don’t require a screwdriver or any other tools to take apart and clean, their suction cups are as good as they get, and they come with almost any connection and fitting you will ever need. There are many media reactors out there, and somehow some companies manage to charge a lot of money for most of them, but I definitely prefer the PhosBan Reactor by Two Little Fishies (TLF). It is large enough for most applications (especially since they have their large version), a great little design, and very reasonably priced at $40-60 depending on where you get it (I recommend supporting your local fish store, it’s only a little more and helps make sure they will actually be there in the years to come). I used three media reactors connected one after the other on my reef. The first housed carbon, the second GFO, and the third held Purigen.

Aluminum-based phosphate-absorbing medias are similar to GFO in that they remove phosphate and silicate, however, they absorb it until they are saturated and then dump it all back, unlike GFO which locks it in. So don’t waste your time or money messing with these. They are white so they are easy to avoid. If you ever open up a container to find ‘the white one’ instead of rusty red/brown GFO, just take it back.

Crushed coral is not usually considered a chemical media, but although it isn’t removing chemicals, it is altering the chemistry. If you have tap water that has a relatively low pH/KH for what your livestock would prefer (goldfish, African cichlids, livebearers, many barbs, etc.) you can safely and effectively use crushed coral to increase the pH/KH. Put it in a filter bag just like carbon. Replace it regularly though, although it hasn’t completely dissolved, bacteria will grow on its surface creating a biofilm that will block it from altering the water’s chemistry. If you decide crushed coral is a good option for your tank, most local fish shops will have it in small bags (5-10 pounds). Rinse it quickly before use. Replace it every month as if it is carbon.

Peat and Peat Granules can be used to alter the water chemistry in the opposite direction, lowering the pH/KH. Peat releases tannic acid and tannins. The tannic acid uses up KH which allows the pH to drop. It also discolors your water, sometimes very significantly (it will look like tea, true blackwater). The fish won’t mind (assuming they are ALL from a blackwater habitat such as the Amazon), but it does look really bad to most people. I personally don’t like using peat, it isn’t permanent and can allow a pH crash. If you need to lower the pH/KH, I suggest 1 – you determine if you really need to (in almost all cases, really good food and water quality will allow any fish to truly thrive) 2 – if you truly need to lower the pH/KH, use half RO and half tap water for water changes. This dilutes the KH which will lower your pH, but still preserves all of the good stuff in tap water (KH, GH, other minerals, etc.) unlike using pure RO. This will produce a lower, but still stable, pH/KH.

Ammo Chips are ammonia-absorbing granules that look like white carbon. The nitrifying bacteria in your filter will do this for you for free, so don’t waste your money on this stuff in most cases. However, if your tank does get thrown off balance (you clean the filters too well which removes too much of the nitrifying bacteria, you add way too many fish at once, you have a long power outage that allows your nitrifying bacteria to die, etc.) you can use this for damage control. Water changes are always essential in these situations though, so ammo chips should only be used in addition to water changes, not in place of. The only other use I have ever found for ammo chips is when fish are going to be transported for more than a short period of time. Since you won’t have filters running and fish don’t stop producing waste just because you move them (in fact, they can produce more because of the stress). putting some ammo chips in the bottom of every bag or bucket can be very beneficial for them. I wouldn’t do this for a short drive, but perhaps anything over one hour would be worth tossing in a little ammo chips.

Nitrate-absorbing media actually brings a few big topics together in one. People obsess over nitrate, so having a media that removes nitrate sounds perfect. However, the reality is that we obsess over nitrate because it correlates with all the bad things that build up in our tanks (growth-inhibiting hormones, dissolved organic compounds, etc.). Nitrate is the only one we can test for with hobbyist test kits. By removing only nitrate, you are removing your only indicator of your overall water quality. You are basically intentionally making the water look better than it really is. Do not use nitrate-absorbing medias. Do water changes. Water changes will remove all the bad things, not just nitrate. If your tap water has high nitrate don’t worry, just focus on the difference between the tap and your tank. That is the increase in nitrate created by your tank, which is the measure of water quality. Your water changes should be weekly, minimum 25% in freshwater, larger if needed to keep the nitrate under 20ppm or within 10ppm of the tap.

Biological Medias

Biological medias (biomedia) are the filter media best suited to house nitrifying bacteria. They are there to do nothing more than provide a physical surface for the bacteria to grow on. Biomedias for submerged use are porous ceramic medias that have a lot of channels and pores in them which create massive amounts of surface area for the bacteria to grow on. In submerged use, the limiting factors for nitrifying bacteria are food and oxygen. Filters provide flow which provides food and oxygen. The surface area of the biomedia provides a surface for the bacteria to grow on where they can sit and allow the oxygen and food to come to them. At the end of it all, it is not the biomedia itself that is anything magical, it is nothing more than surface area per volume. The bacteria are happy to grow on any surface, but they do not simply spread out evenly throughout the tank. Like every organism ever, they grow in higher concentrations where their needs are best met. Organisms concentrate around the source of their limiting factors. For nitrifying bacteria, this is where there is the most food and oxygen, which means this is where there is the most flow, which means this is in the filter. Although the surface areas in the tank (decor, glass, substrate, etc.) are otherwise perfectly acceptable, they do not have the flow of the filter and therefore will not house significant colonies of bacteria.

Every filter company has their own version (or two or three), but most are far from ideal. I prefer Seachem Matrix. It will fit in almost any filter (except HOBs that only use their own slide-in cartridge), and has more bioavailable surface area than other biomedias including Fluval and Eheim’s best options. The difference between surface area and bioavailable surface area is that in medias like Fluval and Eheim that actually have more surface area, most of it is in channels and pores so small that the very bacteria they are meant to house can actually clog the media and block off massive areas of the channels and pores. Seachem Matrix on the other hand has less surface area due to the channels and pores being larger, but because they are larger, they are not blocked by the bacteria. This means all the surface area stays available for bacteria to use. Seachem claims, and some experiences support, that Matrix actually allows for the growth of denitrifying bacteria (the bacteria that need very low oxygen levels and consume nitrate, converting it to nitrogen gas). If it does it will certainly not replace the need for water changes, but even if it doesn’t, it is the best option for a biomedia out there.

Pot scrubbers are something you may hear about on some forums. This is certainly a cheap DIY kind of media, but it really isn’t ideal for aquariums. Its proponents will talk about how it’s nice and cheap and has lots of surface area. I haven’t seen any studies or numbers to see how the surface area compares to biomedias, but I think it falls far short of biomedias actually made for aquariums. In addition, the right biomedia is a one-time purchase. There is no reason to cheap out on something you only have to buy once. The cost affects you minimally once even for the most expensive biomedias. The surface area and how well it hosts bacteria will affect you and your fish for many years. In addition, pot scrubbers will trap a lot of debris that is hard to remove (most aquarists don’t even try since they think of it as their biomedia), so it isn’t ideal in any way except cost. Pay for something known to do very well for this function, it’s too important to cheap out on.

Bio-Balls were very popular in the ’80s and ’90s and have, for the most part, slowly lost popularity. Unfortunately, at least one canister filter manufacturer (Marineland, and probably one or two cheapo knock-offs) has tried to bring them back and decided to use bioballs in their canister filters. This doesn’t make any sense at all. Bioballs are specifically designed for use in trickle filters where their lack of surface area is more than compensated for by the massive amount of oxygen in the air which allows the fewer bacteria to be much more productive. To make it worse, Marineland included a second biomedia, a “porous” media like other canisters. I say “porous” because when the rep showed it to us, I said it didn’t look porous, he said you could break it and see, he tried to, and couldn’t. In the end, they can’t both be the best biomedia so why use one “good” one and one that isn’t as good? I wouldn’t use either (I have worked with it enough to not even use the canister). Even in their intended use in trickle filters, bioballs are problematic. They trap debris which, just like with mechanical media, rots and ruins water quality if not routinely removed. Unfortunately, almost no one routinely cleans bioballs. In addition, there are simply much better sump designs out there these days, making trickle/bioball filters very obsolete. If you are interested in a freshwater sump, please read the following article: Make Your Own Custom Sump.

Biomedia should be cleaned minimally, just enough to remove all visible debris. You can do this under tap water. I know everyone says to rinse it in tank water so that the chlorine doesn’t kill all your bacteria, but I have not found this worry to be justified with any factual data. I have thoroughly cleaned biomedia in tap water routinely for years and never had it produce a single incident where it killed any significant amount of bacteria. You can’t take tap water and sterilize your countertops, it won’t sterilize your biomedia in a couple of minutes (which is far longer than the quick rinse to remove visible debris should ever take). In addition, ‘rinsing’ in still tank water in a bucket doesn’t provide the flow and pressure needed to clean the media quickly and effectively. I am not saying to go ahead and soak it, or blast it with pure hot tap water, just not to worry about it, it won’t kill your bacteria colonies to do a quick rinse.

In general, you should never need to replace biomedia. Just clean it regularly. Of course, the filter media manufacturers are happy to sell it to you every month if you are willing to buy it, but this not only isn’t necessary, it is damaging and will remove your bacteria. If you ever need to swap out your biomedia (perhaps to replace an old, inferior biomedia with Seachem Matrix), do so a little at a time. Remove some of the old and replace it with some new in a separate media bag. Do this a little at a time, 3-4 times, over the course a couple of months. You will now have a superior biomedia without throwing off the balance of your tank. Put the new biomedia first so it has the ammonia and nitrite to eat before the bacteria in the mature biomedia can grab it all.

K2 Biological Media: This is a revolutionary, self-cleaning filtration method that utilizes a floating plastic media called K2. It has been used in waste management and on fish farms for over 10 years, but has only recently been brought to the aquarium hobby. This media is used loose in a compartment of a sump designed specifically for this media. It is partially filled (approximately 60%) with the specially designed K2 media and then strong air stones at the bottom of the section keep the media tumbling and provide superior aeration. This tumbling action helps break down waste and keeps the media itself clean. The media then becomes an ideal site for beneficial bacteria because of the aeration and water flow. The best use for this is in a custom-designed sump. This really is a vastly superior media and filter method. It is very effective at breaking down waste and is self-cleaning. It is hard to get better AND easier, but this media is both. My standard freshwater sump design is made to maximize the use of this superior media and can be found in the Make Your Own Custom Sump article.

K2 Tumbling:

Just to play devil’s advocate for a moment, you don’t need to worry too much about how much biomedia you have. You don’t need to fill a Fluval FX5 with biomedia. In most cases, filling the last tray of a canister is more than adequate. It is actually pretty hard to not have enough biomedia. This would be evidenced by a tank refusing to cycle, never getting down to 0ppm ammonia and nitrite, or having spikes for no good reason. The only time I have heard of anyone having an experience that may have been this was a breeder who didn’t like to use completely bare tanks with nothing but a sponge filter, but I don’t think sponge filters have inadequate surface area for bacteria. Perhaps he waited too long to clean them, at which point the bacteria were growing more on the debris and gunk trapped in the sponge than on the sponge itself, after cleaning the gunk, the tank would be off-balance. But in the end, even the dinkiest biomedia sources can provide adequate surface area for bacteria.


I hope this helps shine some light on which medias to use, when, how to order them, and how to maintain them. You should be able to save money by only buying the media you really need, and know which ones are worth spending a little extra money on. If you have any questions just contact me.

Other Articles You May Be Interested In

Types of Aquarium Filters

Build Your Own Custom Sump

Cycling and Understanding the Good Bacteria

Heaters – The Ticking Timebomb in Every Tank

What You Should Know About Fish Nutrition (Myths and Misconceptions)