How to Choose the Right Filters for Your Aquarium

by Pearl A.

Why You Need Aquarium Filtration || Basic Principles || Understanding Filter Types
Making the Right Filter Choice || Some Final Points about Aquarium Filtration

Aquarium filtration can seem overwhelming when you look at all the filter types available. Many filters claim to be all-purpose solutions, capable of keeping your tank in perfect health and working order by themselves. Retailers reinforce this, as it is (understandably) more attractive to many consumers to buy just one product.

The fact is, your filter choice depends on the size of your aquarium, how heavily it is stocked, and in some cases, the type of organisms you keep. You may be able to get away with one filter, but it’s possible you’ll need 2 or 3. Don’t panic! If you do find you need them, multiple filters don’t necessarily mean high costs. There are low-tech filters out there that make very good secondary choices. But we’ll get to that shortly.

Thoughtfully planning your filtration system ahead of time can save you money and heartache down the road. This article is here to demystify the filter choices available on the market, and to break down basic principles of filtration so you can make the best filter choice for your aquarium.

Why You Need Aquarium Filtration

In a closed environment, small changes have great effects. A fish that dies in the ocean does not change the conditions in the ocean. But a fish that dies in your tank can wreak havoc quickly. The decomposition of organic matter produces chemical by-products toxic to life. Smaller changes, like fish waste, uneaten fish food, even pollution from the air create shifts in pH and other important parameters over time.

Basically, water conditions in nature maintain equilibrium, because ecosystems adapt for that very reason. However, water conditions in an aquarium constantly change due to various natural processes that can’t be avoided. Because of this, aquarium keeping is largely the practice of monitoring and correcting small, everyday changes in your water to avoid greater changes further along. The most important tools that exist for this purpose are aquarium filters.

Basic Principles of Aquarium Filtration

So, filters correct certain unwanted changes to your water conditions. These changes fall into three categories:

toxins in your water from the decomposition of organic matter
dirty or cloudy water caused by physical debris – dirt, excess food, waste
other chemicals in your water caused by pollution from additives, medicines, tap water, the air around your tank, etc.

The first two conditions are completely unavoidable in an aquarium, but the third condition might only arise at times. Together, these conditions bring us to the three types or “stages” of aquarium filtration.

Biological Filtration

Biological filtration is, hands down, the most essential and important type of filtration. Some will take place no matter what, because it is performed by bacteria that exist already in the air, and will settle in your tank once conditions there are right. Biological filtration is often misunderstood, so I’ll try to keep things as simple here as possible. It’s really not confusing, though it is somewhat complex.

You may have seen the term “cycling” while reading about aquarium keeping. The nitrogen cycle is a natural process that takes place in any body of water. When organic waste - excess food, fish poop, dead fish, etc – decomposes, it produces toxic ammonia, deadly to aquatic life. Bacteria convert ammonia to nitrite, then nitrate (also harmful when it builds to high levels).

Nitrate is removed from your water primarily through water changes, although in specialized systems, plants consume it, or "denitrifying" bacteria convert it to harmless nitrogen gas. Nitrate buildup is a problem in any system; freshwater organisms are resistant to levels easily maintained by regular partial water changes, but saltwater organisms are far less tolerant, so saltwater setups often employ additional means of removing nitrates like algal filters (more on this later) or live sand techniques (here's an article for more info).

The Nitrogen Cycle

Ammonia is the most common cause of death in aquariums. Deadly “new tank syndrome” refers to the fact that most beginners introduce fishes into aquariums before the nitrogen cycle has become established. Once the cycle is in place, it can still be disrupted if there are sudden increases in waste (for instance, new fish in your tank) or if your bacteria become depleted due to over-use of antibiotics or changing/cleaning too much of your biological media at one time.

You can use cycle aids to establish bacteria faster, and there are ammonia removers for emergencies that arise. But a stable, healthy system depends on having enough bacteria for the amount of waste produced in your aquarium (the more animals you have, the more biological filtration you'll need). While I mentioned above that bacteria will settle in your tank no matter what, you do need to provide certain conditions to ensure that enough move in and stay put. These conditions are simple:

Appropriate surface materials
Enough surface area
Enough oxygen

Bacteria will not flourish on surfaces like glass or acrylic, because these materials have little surface area. Water flowing past surfaces like these can dislodge microbes easily. Contrast them with the bottom of your tank, probably covered with sand or gravel. It may cover the same dimensions as the walls of your tank, but note how craggy the materials are. These nooks and crannies increase surface area, enabling bacteria to colonize these surfaces with little disruption.

Biological filter media is designed for maximum surface area. You’ll find the same types of media at work in most filters. However, filters differ when it comes to the amount of space they provide for biological media in the first place. If you have a large tank or a heavy bio-load, you need plenty of space for bacteria to colonize.

Last but not least, the bacteria you need for "nitrification" are aerobic: they need oxygen. If there’s a surge in waste, these bacteria quickly deplete the dissolved oxygen in your water, endangering other creatures in your tank. So it's important (as it is anyway) to have good aeration. But it also means filters which keep bacteria separate from your tank - like canister filters or fluidized bed filters - sometimes have problems moving enough oxygen through biological media, and can build up nitrates quickly if a power loss stops filter flow.

Still with me? The main point here is that you need the right amount of surface area for effective biological filtration, which is vital to the health and life of your organisms. The right amount varies according to the biological needs of your tank, and will heavily influence your filter choice.

Mechanical Filtration

Mechanical filtration is the second most important type. Superficially, it removes debris and dirt for clean, clear water you can enjoy looking at. More importantly, it removes particles that could clog your biological filter and interfere with bacterial processes. Thus it is the first “stage”, sometimes referred to as “pre-filtering”.

Mechanical filtration is really quite simple. Most filters perform this function well, although some are better than others.

Chemical Filtration

Chemical filtration removes chemicals from your water. If your water has an unexplained color or smell, it might be due to chemicals. Chemicals enter your tank most commonly through medications. If you medicate in a quarantine tank, as you should, a chemical filter may be unnecessary. But if you end up treating your main tank for some reason, you’ll want a chemical filter to remove medicine afterwards.

Other chemicals, like phosphate and silicate, come from tap water, fish food, or other additives. Phosphate and silicate cause algae blooms, and the best way to remove them is chemical filtration (unless they’re only coming from your tap water, in which case using reverse-osmosis purifiers or water deionizers can also take care of this problem.)

The point here is that chemical filtration has its uses, although you may be able to achieve the same goals without it. Most types of filters include this type of filtration or provide optional add-ons, so it probably won’t affect your filter choice.

Understanding Filter Types

Now let’s break down the functions of different types of filters on the market. I’ll provide details here to help you understand exactly what different filters do (and don’t do), but you can skip these explanations if you’d like and jump ahead to see them summarized in table form.

External Power Filters

External power filters, also called “hang-on” (or HOB) power filters because they hang on to the back or side of your tank, are the most popular filters used by home hobbyists. They are typically rated for aquariums up to 50 gallons in size and suitable for freshwater or saltwater tanks.

Most external power filters include chambers for mechanical and chemical filter media. Some power filters, like Bio Wheel filters or Millennium filters, have great built-in mechanisms for biological filtration. You can achieve success in a small tank with a 3 stage (mechanical, biological and chemical) external power filter. The most common problem with many of these filters is that their small media chambers may not provide enough surface area for an adequate bacterial bed if your tank is heavily stocked.

Canister Filters

Canister filters are external pressurized filters for larger aquariums, upwards of 30-50 gallons. They can be set up underneath or next to your tank, so they're not as limited in size as power filters. Almost all provide 3-stage filtration, as well as various design features that claim to make them the best at all 3 stages. Because they keep their bacterial beds outside your aquarium, they may have problems keeping enough oxygen flowing through in case of surges in waste or power outage.

Canister filters can be great for big tanks and as the technology advances, newer models find innovative ways to deal with inherent problems in design. Compare the features and the amount of space allotted for biological media in different brands.

Internal Power Filters

Internal power filters are similar to external power filters, but are meant to be submerged inside your tank. If you want to use one in a saltwater setup, make sure it is saltwater safe to avoid damage from corrosion. They also tend to be smaller than external power filters, meaning even smaller media chambers.

These filters are popular for low water habitats like turtle tanks. They are also sometimes used to supplement canister filters or external power filters in larger tanks, but their visibility within the tank often makes them less than ideal.

Internal Box or Corner Filters

Internal box or corner filters are inexpensive plastic boxes that rely on air pumps to move water through the media in their chambers. The low flow rate through these filters means they won’t suffice for large or heavily stocked tanks, but they can work as mechanical & chemical filters for smaller setups, although they may still be inadequate biological filters. You’ll have to clean them often, but they are simple to clean.

Sponge Filters

Sponge filters are the simplest filters on the market. They consist of a piece of sponge or foam with an intake tube for an air pump to push water through them. Sponge filters provide good mechanical filtration (sponges are often used as the mechanical prefilter in the powered filters listed above) and are excellent at biological filtration, with plenty of surface area that is easy for bacteria to colonize.

Sponge filters may be the only filter you need in a smaller tank, and they are great for fry (baby fish) tanks (they won't suck up the tiny fry, as other filters can) or quarantine tanks, where chemical filtration is undesirable. Rinsing them gently is all the maintenance you need. However, if you are relying on a sponge alone, mechanical debris may interfere with biological functions. In larger or well-stocked tanks, sponge filters are best for biological supplementation, not primary filtration.

Under Gravel Filters

Undergravel filters are cheap and simple and were far more common in the early days of popular aquarium keeping. These filters consist of a filter plate which sits under your aquarium gravel, and uplift tubes which move water slowly through your gravel and through the plate. They can be driven by air pumps or air stones for heavier flow.

Undergravel filters may not be the best choice in a tank that’s already established, because you’ll have to tear down your setup to install one. But if you’re just starting out and planning a freshwater or simple saltwater setup, these filters may still be a good choice. Plenty of surface area means good mechanical filtration and excellent biological filtration, using the substrates on your tank’s floor for filter media. (Plus, there are optional chemical cartridges for these filters.) Plants, however, may respond poorly to water flow around their roots.

Undergravel filters require regular gravel vacuuming (which you should do anyway) to avoid having to pull them out once installed. Be aware that these filters can develop stubborn buildup underneath that make them a questionable choice for heavily stocked or reef tanks where tear down is going to be considerably difficult. These filters also lose efficiency over time. Opinions about these filters in the aquarium community are divided.

Diatom Filters

Diatom filters are specialized mechanical filters that use a filter media called diatomaceous earth; a sediment made of the skeletons of single-celled microbial algae (diatoms), whose unusual pore structure can trap the smallest particles. Diatom filters are simply the best mechanical filters available. Not only do they provide crystal clear water, they remove algae, parasites, even harmful microbes, keeping your fish healthier, promoting faster healing and greatly improving water conditions.

The drawback with diatom filters is that they become clogged very quickly. Because of this, you shouldn’t use them continuously, so they’re not suitable for primary filtration. However, regular use for short periods is an excellent way to maintain top conditions in your tank, especially if you are having problems with fish health or algae.

Fluidized Bed Filters

Fluidized bed filters house a bacterial bed for biological filtration that won’t have to be disturbed. They suspend fine grained sand in a column of water - sand has great surface area but is a bad choice for most biological filters since water flow easily disrupts it. Fluidized bed filters can be a useful supplement for large aquariums with heavy bio-loads.

You'll also see fluidized bed reactors for chemical media - the same principle suspends fine grained media for increased surface area and contact time, allowing highly efficient chemical filtration, which is not as effective in higher-flow filters. Fluidized bed devices referred to as filters and not reactors are usually the biological type.

Fluidized bed filters are often advised against because they remove oxygen from your system and quickly build up nitrates in power failures. These filters are best for planted aquariums, where they won’t deplete CO2 and oxygen is less of a concern.

Wet/Dry Filters

Wet/Dry filters work by trickling water slowly through mechanical media, building up a good amount of oxygen before passing through the biological filter. This factor combined with greater surface area than canisters or power filters mean these filters are the best biological filters you can find and usually offer (or can be modified for) 3 stage filtration.

Wet/dry filtration, especially within sumps (external reservoirs of water plumbed into your tank) is popular and widely used with sensitive systems like large saltwater or reef tanks; as you might expect, then, it's a bit more complicated than other filters. One benefit of a sump system (there are many!) is that it's easy to setup natural means of nitrate reduction, like algal filters.

Wet/dry filters can be the complete filtration solution for your aquarium, and there is no better biological filter for heavily stocked systems like reefs, but there are challenges involved that make them less suited to beginners.

Conclusions about Filter Types

To summarize before we move on (tank size refers to use as a primary filter):

Filter Type	Tank Size (gal.)	Functions	Flow	Features	Concerns
External Power Filter	Fresh/Salt up to 50	Mechanical & Chemical with some Biological	Mid to High Flow	Hang-on	Small surface area for media
Canister Filter	Fresh/Salt 30 - 400	Mechanical & Chemical with some Biological	High Flow	Larger capacities	Oxygen flow for biological may face problems
Internal Power Filter	Fresh up to 30 (some to 50)	Mechanical & Chemical with little Biological	Medium Flow	Submersible, good for turtle tanks, or dead spots in large tanks	Many are freshwater only; small surface area for media
Box or Corner Filter	Fresh/Salt up to 30	Mechanical & Chemical with little Biological	Low Flow	Submersible, cheap, easy to maintain	Frequent cleaning; small surface area for media
Sponge Filter	Fresh/Salt up to 100	Biological & Mechanical	Low Flow	Cheap, easy to maintain, fry-safe and quarantine-safe. Good secondary.	Use prefilter to avoid frequent clogging
Undergravel Filter	Fresh/Salt up to 200	Biological & Mechanical w/optional Chemical	Low Flow	Cheap, easy to maintain. Large surface area.	Bad for planted tanks, lose efficiency over time, potential maintenance issues
Diatom Filter	Fresh/Salt	Mechanical	Medium Flow	Best mechanical. Removes algae, bacteria and parasites.	Maintenance only, not for continual use. Clogs quickly.
Fluidized Bed Filter	Fresh/Salt up to 900	Biological (planted tanks & heavy bio-loads), or chemical ("reactors")	Low Flow	Biological: Undisturbed bacterial bed. Won’t remove CO2 needed by plants. Chemical: most efficient.	Biological:Removes oxygen from water, nitrate hazard during power outages. Chemical: may be unnecessary
Wet/Dry Filter	Fresh/Salt, customizable (reefs & large saltwater)	Best biological, mechanical, sometimes chemical	Low or Medium Flow (varies)	Unmatched efficiency and power, additional benefits.	May require plumbing or additional equipment/planning. Advanced.

Making The Right Filter Choice

If you’ve done additional research about aquarium filters, you know that there are many differences in opinion. It’s likely that a number of filter configurations could work for your setup. But there will always be certain choices that are wiser than others!

All you need to make a reasonable, informed decision is an understanding of the basic principles of filtration and of the basic properties of filter types on the market. Hopefully you now have a greater handle on both of these topics.

Powered filters have higher flow rates and thus are good primary filters, especially in larger tanks. You may find that using only an undergravel or sponge filter works for low flow setups, but avoid undergravel filters in planted tanks.

If you have a small fresh or saltwater tank that isn’t heavily stocked, you may be able to get away with using only an external power filter, especially one with a bio-wheel or wet/dry compartment. However, it is inexpensive and easy to add a sponge filter to your setup. The same holds true for canister and internal filters, powered or not.

The point here is that any filter with a limited space for biological filtration is going to be limited in its capacity. While some brands promote designs that have greater surface area for better biological filtration, be prepared to supplement them anyway if your conditions seem less than optimal. You should always closely monitor ammonia, nitrate and nitrite levels in your aquarium to understand how well your biological filter is working and determine whether you need additional surface area.

If your budget allows it, you can’t really go wrong with a diatom filter on top of whatever primary and/or secondary filtration you choose. It is a great tool for maintenance and will give you crystal clear water, ease the strain on your other filters and prevent all kinds of illness and nuisance before they have a chance to affect your aquarium’s inhabitants – especially if you don’t have a protein skimmer.

There's some debate about fluidized bed filters in the aquarist community. Fluidized bed filters offer efficient biological filtration, but their tendency to remove oxygen is a concern (unless you have a planted tank); while fluidized bed reactors for chemical media are powerful chemical filters but probably not necessary in most situations.

Wet/dry filters are the most powerful biological filters you can find, especially popular for reefs and heavily stocked saltwater systems. Not generally recommended for beginners (then again, neither are reef tanks), it's worth your while to do some further research into the different ways hobbyists use wet/dry filters and sumps successfully. Check out my short article on wet/dry filtration, and don't forget there are many online aquarium forums to help!

When in doubt, add a sponge! They're easy to clean, cheap to buy and replace, and unbreakable.

Some Final Points about Aquarium Filtration

The larger your tank is, the more forgiving your water conditions (but larger tanks need more circulation and other considerations, so don't go overboard).

Buy a larger filter than is recommended for your tank size (with the exception of undergravel filters, which are sized to fit the bottom of your tank) to keep filtration efficient.

If you have a saltwater aquarium, using a protein skimmer greatly lightens the load on your filters, and generally improves your water conditions.

Proper circulation is not only important for proper aeration and trace element distribution, it is key to keeping water and oxygen flowing through your filter in large tanks.

Regardless of the filters you use, regular partial water changes and gravel vacuuming remain essential to the health of any aquarium. It comes with the hobby, so there’s no use trying to avoid it!

Don’t neglect regular filter cleaning as part of your routine – there are brushes just for this purpose to make it easy.

Clean or change filter media according to directions, or they won’t be effective - but never clean or change all your biological media at once, to preserve your bacterial bed (gravel vacuuming is ok).

If you have a filter with only one compartment for biological media, divide your media into several filter bags so you can easily do partial changes.

Always monitor crucial water parameters, especially ammonia and pH, regularly to avoid major problems/losses.

Avoid using filters or other aquarium equipment not produced by recognized industry manufacturers. Years of testing, design improvements and advanced technology are worth a few extra bucks and will save you more money and heartache down the road. It’s also worth something to be able to find reviews of products you are interested in buying.