What to know about the 3 main types of filtration for aquariums
Your aquarium is a closed system that needs your help to stay clean and habitable for any fish or plant life inside. The filtration system is the workhorse of your tank maintenance regimen, running constantly to clean and purify the water of contaminants. But if you’re new to the hobby, all the filtration options out there can seem overwhelming. There are actually three main types of aquarium filtration—biological, chemical, and mechanical. Having an understanding of what these types of filtration are, how they work, and what purpose they serve can help you to choose a filtration solution that best serves your aquarium setup.
What types of filtration do you need?
Aquarium filtration is essential for both freshwater and saltwater tanks. Here’s a quick breakdown of the three main types of filtration you can expect to use in your tank:
Biological filtration is an absolute must. No matter what type of tank you have, biological filtration is critical for maintaining consistently low levels of ammonia and nitrites while also having controlled levels of nitrates to keep your tank inhabitants healthy.
Mechanical filtration is also important for everyday filtration because it allows you to remove solid wastes before they can decay and cause ammonia spikes or other imbalances and prevents the clogging of your filtration media.
Unlike the other two types of filtration, chemical filtration is not an everyday filtration solution; instead, think of chemical filtration as a tool that is used only occasionally to resolve very specific issues or to control the water chemistry to a very specific set of parameters.
The reasons behind these guidelines will become clearer as we take a deeper look at each type of filtration.
Biological filtration
Like any living things, the plants and animals in your tank produce wastes that can be toxic to the ecosystem if allowed to accumulate. Nature combats this problem and maintains consistent water quality through biological filtration. Biological filtration is a process that uses beneficial bacteria to break down toxic nitrogen compounds, which is called the nitrogen cycle.
Types of biological filtration
There are two types of biofiltration: aerobic, which occurs in the presence of oxygen, and anaerobic, which occurs only where there is little or no oxygen present. 
Aerobic biofiltration is by far the most common form of biological filtration used in aquariums, as it is critical for both freshwater and saltwater tanks. It leverages nitrifying bacteria that use oxygen to break down harmful ammonia and nitrite compounds and produce comparatively fewer toxic nitrates. Nitrates can then be taken up by plant life, diluted during water changes, removed through use of chemicals, or further degraded by anaerobic biofiltration into Nitrogen gas.
Anaerobic biofiltration, on the other hand, requires anoxic zones in your tank. These are areas where little or no oxygen is present, such as deep inside porous rock or inside certain types of filter media. The process uses denitrifying bacteria that break down nitrates to form nitrogen gas that can then exit out of the tank. Anaerobic biofiltration is especially important for saltwater tanks because marine fish, invertebrates, and corals are particularly sensitive to nitrate levels. That’s why saltwater aquarists may take extra steps to create anoxic zones in their tanks—by adding a deep sand bed or by using specialized biomedia, for example. Freshwater fish, on the other hand, are often hardier and less sensitive to nitrate levels, unless they experience a quick change in nitrate concentration, therefore denitrifying bacteria are helpful but not as critical for most freshwater tanks. But again, for a healthier tank they should be considered.
How to use biofiltration in your tank
Beneficial bacteria will naturally colonize any available surface in your tank; however, you’ll still likely need some type of biological filter (or biofilter) device to ramp up bacterial activity to the point that it is adequate to keep your water healthy. At their simplest, biofilters are devices that contain some type of media substrate where beneficial bacteria are allowed to grow. Biofilter media can be made out of a variety of materials, such as gravel, sand, sponge, plastic or man-made ceramics. The key to a good biofilter media is ample surface area to maximize the numbers of beneficial bacteria present and high porosity to allow the water to get into the media and access the surface area. This allows for as much nitrification activity as possible.
Some biofilters work by using an air or water pump to move water through your chosen filter media, which serves to both aerate the water, and expose it to the bacteria living in and on your filter media. While other biofilters are more passive, allowing for more anaerobic bacteria to grow as oxygen levels inside the media are allowed to drop.
Listed from most basic to most advanced, some types of filters for biological filtration include:
- Box filters
- Power filters or hang-on-back (HOB) filters
- Canister filters
- Trickle filters (wet/dry filters)
- Fluidized bed filters
These filter units have a cavity or container that you can pack with your chosen biomedia. Of these, trickle filters offer the best water aeration, and are more often used in freshwater aquariums, ponds and in high bio-load systems such as in aquaculture.
Two other basic filter options, under-gravel filters (UGF) and sponge filters, also offer some degree of biological filtration, but are often not adequate for tanks with high populations of fish due to their limited surface area. Even though these filters offer comparatively less biological filtration efficiency, the filtration of the aquarium, as a whole, can be boosted with a separate biofilter media located in another area of the tank. Natural biofilters could be deep sand beds, naturally occurring rocks or man-made ceramics placed anywhere in the system.
What is MarinePure® biofilter media?
MarinePure is a man-made ceramic biofilter media that offers a unique structure that allows water to flow easily through its interconnected pores. As a result, MarinePure maximizes usable surface area, resists clogging, and supports the growth of both nitrifying and denitrifying bacteria, which can help to bring greater stability to the nitrogen cycle in your tank. Choosing a highly efficient biomedia like MarinePure can bring several benefits, like allowing you to do less frequent water changes, house more fish, or just maintaining a more consistent water chemistry. If you would like more information on MarinePure biofilter media please feel free to reach us out at info@cermedia.com.
Mechanical Filtration
Mechanical filtration is a process that uses filter media to physically screen out solid particles from aquarium water. This is important because it not only helps with water clarity, but it also allows you to remove uneaten food, digestive wastes from fish, and dead plant or animal material whose proteins will break down into ammonia as they decay.
Mechanical filtration must be used in combination with other filtration types since it can’t address dissolved wastes. For this reason, mechanical filtration is typically used in conjunction with biofiltration.
Types of mechanical filtration
When it comes to mechanical filtration, you’ll encounter it mostly as a first-stage pre-filter for removal of large particles ahead of other filtration media. This is important for both saltwater and freshwater tanks, as it helps to prevent clogs in the biofilter media, decreasing the workload on the beneficial bacteria and helping to maintain more consistent water quality.
The other, less common type of mechanical filtration is diatomic filtration. This type offers extremely fine filtration, and is used to capture very small particles, generally to resolve occasional issues like algal or bacterial bloom, or for improving water clarity.
How to use mechanical filtration in your tank
Nearly all types of filter units accept mechanical media. The most basic types of media include filter socks or filter sleeves that are designed to be fitted on the water intake for your filter. HOB filters and power filters often have specific internal cavities dedicated for mechanical filter media or cartridges. Canister filters have more customizable filtration options, and you have some flexibility in selecting your filtration media.
Depending upon your filter and your chosen media, you may have one or more stages of mechanical filtration. This is achieved by arranging your mechanical filtration media in order so that your water flows through from the coarsest to the finest filter media. Mechanical filters may come in the form of a filter pad, filter floss, or sponge, and are typically made from synthetic materials such as foam or polyester fiber.
Chemical Filtration
Chemical filtration is a process used to remove dissolved substances through a chemical reaction, such as absorption or adsorption. As such, it is a precision filtration option that is mostly used to target specific contaminants.
Types of chemical filtration
Chemical filtration is based on an attraction between the filter media and a targeted contaminant; therefore, the “type” of chemical filtration depends entirely on the selected media. Common types of chemical filtration media, and their uses, include:
- Activated charcoal works by removing contaminants through adsorption. It is commonly used to remove odors, colors, tannins, dissolved organic compounds, chlorine, chloramine, and medications.
- Granular ferric oxide (GFO) is used to reduce phosphate and silicate levels, typically to control algae growth, though it can also be used to remove fluoride, selenium, arsenic, and other metals. GFO is more commonly used in reef aquariums, where algal growth can be problematic.
- Zeolite is used to absorb ammonia. It is useful for quickly improving water quality in the case of an ammonia spike or other issue. It is worth noting that zeolite is recommended only as a temporary fix since it can become saturated quickly, while biofiltration is more appropriate for routine control of ammonia.
- Resins are used for removal of organics as well as nitrates, and other contaminants. There are many proprietary resins on the market, so take care to select a product that meets your specific needs.
- Peat moss is used to reduce or stabilize pH by releasing humic acid.
How to use chemical filtration in your tank
The most basic means of applying chemical filtration is through the use of a filter media bag, which involves placing your chosen filter media in a permeable bag, then placing the bag in an area of the tank where there is good water flow. Once the issue at hand is resolved, then you simply remove the filter bag.
Another device, known as a media reactor, is comparatively more mechanically complex, but delivers faster results. Media reactors are usually a supplemental filtration device that use a pump to force water into a reactor containing your chosen chemical filter media. These units are typically equipped with flow rate controls to determine contact time, and they therefore allow for more complete contact with chemical media than can be achieved with a media bag. Media reactors are mostly used in saltwater tanks because marine fishes and reefs are so sensitive to contaminant levels. They can be used for freshwater tanks too, but are typically unnecessary.
In any case, aquarists seeking to use chemical filtration need to take care to measure and use the right amount of filter media depending upon tank size and contaminant levels.