Aquarium Filter Types

Filtration is one of the most important aspects of aquarium care. It is an investment, which means that although it often has a high initial cost, it will pay you back in the long run. It is easy to understand that the extra money invested into a better filtration system could pay for itself by the fish it saves or the time it saves you in maintenance. There is a vast variety of filters out there, and among each type is a huge array of filters from various companies. Filtration itself needs to be understood before the filters are looked at.

Mechanical Filtration

Mechanical filtration is the physical collection of particulate matter from the water. It will trap physical debris and hold it until you clean the filter. Even once it is trapped, that debris still has a negative impact on the water. As it breaks down, it becomes nitrate, phosphate, and other harmful chemicals that build up in the water and lower water quality. So although a filter may not be so clogged that it has reduced flow or stopped running, it is still significantly decreasing water quality. So the water stays crystal clear, giving the impression that the filter is still working fine even if it is having a negative effect on the water quality and therefore the fish as well. This is why routine filter maintenance is a must, even if things look fine.

Different medias for mechanical filtration will trap different sized particles. Large course media should be the first thing water encounters in the filter. After that, you can have fine mechanical media. There is no point in adding courser media in the filter where the water will have already gone through a finer media because it will not catch much, if anything, and will become a media for nitrifying bacteria. Every tank is different so in some tanks, the fine mechanical will collect a lot of debris while the course mechanical media seems new and clean, in other tanks the opposite will occur. The ideal mechanical media is in a filter that is under pressure or sealed. When under pressure in a sealed filter, the water is forced through the media. In filters that are open, the water goes through the mechanical media under the force of gravity alone. Because it is not under pressure, the water simply goes around or over it when the media is clogged. This means that although the filter is running, the water is not being cleaned well because it is bypassing the mechanical media.

Biological Filtration

Biological filtration is the most important. An efficient biological setup is essential to any system. Nitrifying bacteria that consume ammonia and nitrite will live on surfaces where the conditions are ideal. That means the ideal temperature, oxygen level, and food supply will favor these bacteria. In an aquarium, the temperature is generally uniform, so this will not affect where the nitrifying bacteria will colonize. Oxygen levels and food availability will greatly affect the nitrifying bacteria. Sitting in a corner on the glass or on the gravel will not supply them with the same amount of food and oxygen as being in a biological filtration media where there is a constant high flow of new nutrients and oxygen. This is why the nitrifying bacteria will tend to colonize in the filter on the high surface area that biological filtration media provides.

There are many forms of biological filtration media. Surface area is a must. The bacteria need to be exposed to the water so they have to be in thin layers, almost a single layer thick, in order for them all to live. The high flow will bring in the nutrients for them, so as long as there is a decent flow rate, that will be taken care of. Oxygen levels are frequently the limiting factor for how efficient the biological filtration will work. If the oxygen level in the water is high enough, canister filters and other submerged media filters can be very effective. But these usually require a lot more surface area to compensate for the relative lack of oxygen in order to be as effective as other forms of filtration. Wet-dry and trickle filters that expose the surface of the media directly to the air are a very efficient way of ensuring high oxygen levels because there can be up to 30,000 times as much oxygen in the air as in the water. By briefly exposing the bacteria to this oxygen-rich environment, their efficiency greatly increases. Cleaning biological media needs to be kept to a minimum to prevent the loss of nitrifying bacteria. This usually means lightly rinsing the media with tank water when debris deposits are visible on the media and nothing more.

Chemical Filtration

I do not consider chemical filtration as a main part of filtration because it is not an essential part of an aquarium. It can be used to remove odor or discoloration, but in either of these cases, it is treating a symptom and what you need to deal with is the cause. The only other use for chemical filtration is the (hopefully rare) removal of medications. When chemical filtration (carbon) is needed, there are some things that need to be understood.

Carbon effectively grabs many types of chemicals in the water, from medications to trace elements. There is a wide range of quality levels in carbon. In general, you will get what you pay for, so the highest quality carbon will be more expensive. Carbon does not last forever. The lifespan of carbon is dependent on a few things including the quality and quantity of the carbon used, the amount of chemicals in the water, the flow the carbon receives, etc. The best practice is to replace carbon on a monthly basis during your monthly filter cleaning. I do not use carbon on a regular basis because it can remove some beneficial substances such as trace elements that fish, corals, and plants need. If proper water changes are carried out, the possible consequences of not using carbon should not come into effect. Many people will use carbon for 24-48 hours once per month. This removes the bad stuff without constantly removing the good stuff.

If you do enough water changes, you shouldn’t need carbon.

Filter Types

There is a huge number of filters on the market, all with their pros and cons. There are three things that you should be looking for: efficient biological filtration, effective mechanical filtration, and the availability to use carbon when it is needed. When these are equal, you can look at ease of use and cost.

Not every tank is the same. So what may be ideal for a large highly stocked Lake Malawi mbuna tank may not be ideal for a small lightly stocked planted community tank. Because of this, there is no ‘best’ filter out there for every tank.

Sponge Filters

The simplest filters are internal sponge filters. They are powered by air and are a good biological and mechanical media in one. These are best for simple breeding setups where a single pair or small group needs to be isolated for breeding, or for raising their fry. There is no money spent after the purchase of the filter itself, which makes the overhead or long term cost of the filter nothing but the electricity to run the air pump which you should have anyway. Routine cleaning is a must, which is nothing more than squeezing out the sponge itself in old tank water after each water change (or at least monthly). If a tank is too heavily stocked or requires a lot of mechanical media, then these filters are not the best option. One of their biggest downsides is that they have to sit inside the tank, making them an eyesore unless hidden well by a tank’s decor. I also don’t like to have to go into the tank and bother the fish just to maintain the filter, which is a requirement with sponge filters.

Best for: Sponge filters are best for smaller tanks, up to about 29 gallons, housing shrimp, fry, or small community fish (all low bioload).

My Favorite: Any sponge filter that will suction cup to the back of the tank. This keeps them in one spot and out of the way. The suction cup ones also have an adjustable output so that you can adjust how tall the output pipe is as well as what direction it sends the flow.

For the air pump, I really like the Whisper Air 100. I still remember when they first came out and all air pumps were noisy back then. The sales rep plugged it in and we thought it wasn’t working because it was dead silent. They’re silent and strong. I always go with a much larger air pump then I think I might need because I hate to buy something like that and then have to buy another one because the first one was slightly too weak. I can easily cut back the flow if needed. If it’s strong enough, I can even run multiple tanks on one air pump.

Internal Box Air Filters

These filters are usually a small clear plastic box that is air powered and has a cartridge similar to that on a hang-on back power filter or is open inside for you to use whatever media you want to use. Because the cartridges usually need to be replaced, at least long term, these filters are not as good as the sponge filters at conserving nitrifying bacteria when cleaned. They have a low initial cost but require media replacement. Their relatively small capacity and the space they take up make them less than ideal in almost any situation. Basically, you have the same maintenance and media cost as a HOB, but with less flow and the hassle of needing to go in the tank to do any maintenance.

I do not recommend these for any tanks.

Under Gravel Filters (UGFs)

These are an all-time classic favorite for many aquarists. They are a perforated plate that sits on the bottom of the aquarium, under the gravel. There are a few variations to this setup. One includes air-powered riser tubes. The second involves a powerhead or other pump to power the riser tubes. The third uses a powerhead to push water down the tubes, this method is called reverse under gravel filtration (RUGF). Some even hybridize this with other filter methods and attach the riser tubes to hang on back or canister filters. This filter uses the gravel itself as a media. The gravel functions as a biological media as well as mechanical media. The high surface area of the gravel provides ample space for nitrifying bacteria. Because of the gravel functioning as a mechanical media, it will trap a lot of debris. This requires the aquarist to keep up with water changes that always include a complete gravel vacuuming in order to attempt to keep the gravel free of debris.

The problems with UGFs are significant though. Two major problems occur with too much debris in the gravel. The first is that debris can build up and actually choke out the nitrifying bacteria and cut off their oxygen supply. The second problem is that debris buildup can lead to excessive nitrate production as the debris slowly breaks down. The plate of the under gravel filter itself will allow debris buildup on the bottom of the tank, out of reach of the aquarist and the gravel vacuum. Some people claim that you can simply run the siphon tube down the riser tubes, but this only removes the debris within a couple of inches of the riser tubes, if that. All this debris building up leads to an excessive nitrate level.

Many fish are diggers and since water takes the path of least resistance, unless the gravel is the exact same depth throughout the tank it will not evenly go through the gravel. Once a fish digs down close to the plate, the under gravel filter itself is almost useless. This means that the effective area for nitrifying bacteria is only a fraction of all the gravel. Even with a perfectly level gravel layer, the water is not going to pass through the gravel evenly, it will pass through the gravel immediately adjacent to the riser tubes. Again, allowing water to bypass the vast majority of the gravel.

UGFs are not compatible with sand and other very fine substrates, which are a better substrate than gravel anyway. Considering that UGFs should also have another filter on the tank in addition to the UGF and the problems that they can cause, most aquarists have left them behind in the ’80s and moved on to other types of filters that are better in every way.

Do not use UGFs in any tank.

Internal Power Filters

These are usually an elongated media chamber with a pump at one end that sits inside the tank. These can provide good mechanical filtration as well as biological depending on their media layout. These require that the aquarist actually goes into and disrupts the tank to clean the filter, which I hate. They can also be hard to tell when they need to be maintained depending on the design. These use up valuable in-tank space that the fish could use and can be a huge eyesore if not hidden well by the tank’s decor.

Best for: These are best reserved for tanks with low water levels such as those used for turtles, frogs, etc. In other tanks, HOBs and canisters will be much better options.

My Favorite: I like Taam/Rio because they have a great pump and multiple media chambers making them effective and easily customized.

Hang On Back Power Filters (HOBs)

These usually include a box that hangs off the back of the tank with an intake tube going into the water. They use effectively no space inside the tank. They provide moderate surface agitation depending on their exact design and the tank’s water level. Most are not very customizable as far as media goes though. They are generally moderate in cost. Most require a continued expense in cartridges. Some come with carbon built-in so there is no way to not use it, others come with it included, but not built-in so you may or may not use it. Many HOBs have separate biological media. This may involve what is equivalent to a course mechanical media after finer mechanical media. Others include plastic plates and even paddle wheels exposing the biological media to the air. These filters are easy to maintain, but some designs may take longer. There is generally a relatively low and fixed media capacity with most designs. Most HOBs use a slide-in cartridge that usually contains the mechanical filtration and the carbon. Many of these actually have the carbon sealed within the mechanical media. In addition, most of these cartridges are not interchangeable between brands, so the hang-on-back filters are not nearly as customizable as canisters.

Best for: HOBs are great for a wide variety of tanks, everything from a 10-gallon with small community fish to a 75-gallon or larger with cichlids or goldfish.

My Favorite: AquaClear filters have more flow and a massive amount of media compared to any other HOB filter out there. The only real downside is the intake strainer which is much smaller than it should be. I have started using my own DIY intake strainer made with black mesh (needlepoint canvas), which you can see here: AquaClear Intake Cover. Don’t waste your money on any HOB other than the AquaClear. The difference is so drastic, even against Bio-Wheels, that the others are simply not worth their cost. The AquaClears will pay for themselves in the cost savings in filter cartridges alone. Just so you’re aware, AquaClears are sold as both AquaClear and Fluval. In addition, there is another Fluval HOB, the C-series (C2, C3, etc.). I’m talking about the original AquaClears, not the Fluval C-series Update: The Seachem Tidal is a newer HOB with a design comparable to an AquaClear but the motor is inside the intake tube so that it is submerged in the water. This makes it much more likely to come back on without any issues after a power outage. I still prefer the AquaClears, but the Seachem Tidal is at least a great second option (many people consider it as good or even better than the AquaClears).

Canister Filters

Canister filters are a large canister chamber that sits under the tank. Hoses connect it to the main tank. The canister is gravity fed and a built-in pump returns the water to the tank. These are sealed systems that are under pressure. These involve a large volume that can be used for an assortment of media. Most use trays to divide the canister into separate sections, each of which can include a different media. These provide a very large media capacity which allows them to handle large tank volumes very efficiently. They require a moderate amount of time to maintain, depending on the amount of debris collected. They are one of the more or most expensive types of filtration. Many include or allow for media that can be easily cleaned and reused, greatly reducing continuous cost. Their customizability is unmatched since anything at all can be placed in the trays. Almost all manufacturers provide media specific to their canisters, the mechanical media fits the trays very well and works very effectively. Canisters are unmatched when it comes to mechanical filtration because they are closed. With open filters, as soon as the mechanical media is partially blocked, it will flow over or around it, completely bypassing it. Since canisters are closed, they don’t allow this at all. This allows them to keep working even as the media gets clogged up (to a limit) and allows you to use finer mechanical media.

It’s important to note that all filters should be cleaned monthly. There’s nothing magical about the inside of a canister that makes it so they don’t need to be cleaned. They collect the debris for us to remove it from the system. If we don’t, it just rots and lowers water quality. There’s nothing beneficial about letting it get so clogged up that it can’t even move water. The idea that they don’t need to be cleaned is nothing more than an excuse for laziness. The good news is that if you actually clean them every month, it’s very quick and easy (as little as 10 minutes).

Best for: Canisters are great for anything from about 55 gallons and up. On a 55-65, I might do a HOB depending on the fish. At about 75 and up, it’s a canister hands down for me.

My Favorite: Fluval are by far the best canisters I’ve used. They’re strong, silent, hold a ton of media, have great hardware, and last a long time. They are not the cheapest, but I’ve used or had clients with probably every other canister on the market and there’s a reason I buy Fluvals when I’m spending my own money on them.

For some reason, some people like to waste a lot of money on canisters and end up having 2-3x the filtration they actually need. This in and of itself isn’t really a problem unless it creates too much flow. But it is a big waste of money if there’s no benefit to the additional filter. For a 40-65 gallon, I’d do the 407. On 75-110, I’d do the FX4. At about 125 and up, I’ll do one FX6. This is when some people get crazy, I’ve seen people recommend as much as 3 FX6s on a 180. These people are out of their minds. They’re either selling them to you, have never used them even close to their full potential, or just have a ton of money they are trying to get rid of. I’ve had one FX6 handle a HEAVILY stocked 220 and a moderately stocked 300. So the idea of putting two or three on a 180 is laughable/scary. So if people saying you need two FX6s on a 125 or anything like that was your reason to avoid canisters, definitely reconsider.

Sumps

Sumps are a second tank that sits inside the stand that the display tank drains into. They can be a good option for large systems (usually at least 75 gallons). There are manufactured sumps or you can build your own. Sumps increase the volume of the system and are extremely customizable (at least if you’re building your own), and give you quick access to the media (compared to a canister). You can use whatever size you want, whatever pump you want, whatever media you want, etc. There are a lot of bad designs out there though, so be very careful if you are going to use a sump. I know some people will only consider sumps on anything bigger than about a 75, but having done it both ways, I highly prefer a good canister over a sump. They are easier (no need to plan and build every aspect of it), have better mechanical, and are usually cheaper (even the good ones, such as Fluval). For more information on sumps, including pros and cons of sumps vs canisters and details on making your own sump, read the article Build Your Own Custom Sump.

Wet-Dry/Trickle Filters

These are usually a section of a sump. They are a chamber filled with biological media such as bioballs that the water slowly trickles over, exposing the nitrifying bacteria to the water and air. These frequently include separate mechanical and chemical media chambers. The biological media can collect debris, which like any other filter media that is not routinely cleaned well, can cause excessive nitrate and phosphate and lower water quality. So these do require routine breakdown and rinsing to keep the media clear of debris while preserving the nitrifying bacteria, frequently being every one to six months. These are generally used on large systems with large bioloads and relatively sensitive fish. These are rarely used anymore since there are much better media options for modern sump designs.

Filter Sizing

I have found that the best way to determine filtration needed is to simply cut whatever the ‘up to’ rating is in half. For example, if a filter claims it can handle ‘up to 100 gallons’ I would only count it as about 50 gallons worth of filtration. This provides enough filtration for a moderately stocked community type tank. High stocking levels or high bioload fish such as goldfish and cichlids will need even more filtration. I ignore gallons per hour (gph) ratings for the most part, but overall, more is better. Focusing on flow too much can lead to a lot of movement without much cleaning. Remember that the filters are not cleaning x many gallons, they are cleaning up after a certain bioload. Moving water doesn’t clean it, flow combined with media capacity cleans water.

Remember that all the filtration in the world can be undone by insufficient water changes. Filtration does not and can not compensate for a lack of water changes in any way.

Summary

In most cases, I’ll use sponge filters on tanks up to about 29 gallons, a HOB on tanks around 29-65 gallons, and a canister on anything about 75 gallons and up. When in doubt, go with the larger one. Clean them every month.

Other Articles You May Be Interested In

Filter Media Types Guide

Carbon Filter Media in Aquariums

Build Your Own Custom Sump

AquaClear Intake Cover

Water Changes and Water Quality

Stocking an Aquarium

Sand as a Superior Substrate

Heaters – The Ticking Timebomb in Every Tank