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 are a vast number of types of filters out there, and among each type is a huge array of filters from all sorts of 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. While it is in holding that debris still has a negative impact on the water. As it breaks down it will end up increasing 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 because it is bypassing the mechanical media.

Biological Filtration

Biological filtration is the most important to the fish. 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 quite 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 other forms of biological filtration 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 in to effect. Many people will use carbon for 24-48 hours once per month. This removes the bad stuff without constantly removing the good stuff.

Filter Types

There is a huge number of filters on the market, all with their pros and cons. There are three things that you are 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 highly stocked Lake Malawi mbuna tank may not be ideal for a lightly stocked community tank. Because of this, there is no ‘best’ filter out there for every tank. There is one important issue to keep in mind, the ability to customize a filter. By being able to customize the filter you can meet different or changing needs. The best filter type for this is the canister filter. Most now have multiple trays that allow you to use whatever media you need to. Best of all is that you do not have to have an exact media. This means that you can use another company’s media (or a cheap generic media) instead of one that has has been made to fit that exact filter (and costs more). Most Hong-on-back filters 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.

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 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, usually involving rinsing the debris from the sponge with tank water to conserve the nitrifying bacteria. I just have a bucket half full of water from a water change and squeeze the sponge clean in the water. 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. However, these are perfect for shrimp tanks, fry tanks, breeding setups, and especially when you will be running many tanks since one large air pump can run all your sponge filters.
My Favorite: Azoo Sponge Filters

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. 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.

Under Gravel Filters (UGFs): These are an all-time classic favorite for many aquarists. These are composed of a perforated plate that sits on the bottom glass 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. 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.

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 his gravel vacuum. Some people claim that you can simply run the siphon tube down the riser tubes, but in my experience, 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.

UGFs are not compatible with sand and other very fine substrates, which are a better substrate 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 and moved on to other types of filters.

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. 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. 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: Fluval 4plus (discontinued many years ago) or Taam/Rio

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 do not use much room in the tank at all. 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. These 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.
My Favorite: AquaClear filters hold more media and have more flow than 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.

Canister Filters: Canister filters involve 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 efficiently.
My Favorite: Fluval

Sumps: Sumps 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, especially when you make your own. 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 used to never recommend them because I could never rely on them alone, I always had to supplement them with a good canister. However, I have now seen that a properly designed sump can run a tank on its own (although for freshwater I still prefer a good canister). For more information, read about how to Build Your Own Custom Sump.

Wet-Dry/Trickle Filters: These generally involve a large chamber filled with biological media such as bioballs that the water slowly trickles over, exposing the nitrifying bacteria to the water and air. These are usually part of a sump that is gravity fed and sits under the tank inside the stand. The water collects in the sump where it is then pumped back up to the tank. These frequently include separate mechanical and chemical media chambers. The biological media can collect debris, which like any other filter media if not routinely cleaned well can cause excessive nitrate and phosphate concentrations. So these do require occasional 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 with more 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 waste producers 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.

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.