Aquarium Water Filtration Methods & Types

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The aquarium hobby is filled with an abundance of filter gears, gadgets and equipment and deciding what to buy and what to skip can be really frustrating. The prices also vary considerably, even between seemingly identical filters. It is impossible to give any specific recommendations about what to buy and what to leave out since different aquariums require different types of filtration. In these articles, I will try to briefly explain how some of the most commonly used types of equipment work so that you will be more prepared when you start making decisions regarding your specific aquarium.

As mentioned above, different setups need different filter systems.

  • Sturdy fish need less potent filtration than delicate fish.
  • Carbon dioxide craving plants can dislike certain types of filtration.
  • The creatures found in a saltwater aquarium will normally need extremely high filtration to keep the levels of soluble waste down.
  • Generally speaking, a small aquarium will need more powerful filtration than a large aquarium, since a large water mass will dilute organic waste.
  • A crowded aquarium will need stronger filtration than a lightly stocked aquarium since more waste is produced in the first one.
  • A planted aquarium will need less filtration than a barren aquarium since plants use organic waste as nutrition, thereby removing it from the water (at least temporarily).

The type of filter systems that are available for you will also vary. In a freshwater aquarium, you will, for instance, be unable to use protein skimmers. If you have a breeding tank with really small fry, it is not a good idea to use a filter that will suck in the fish babies. When your Gourami has started to build a fragile bubble nest, you can not use a filter system that creates violent water movements. The list goes on and on. The only way to find out which type of filter or filters that are ideal for you is to read more about the fish species you intend to keep.

Your budget and the amount of time you are prepared to spend on your aquarium are two important factors when choosing filter equipment. It is often possible to save money by tolerating noisy or ugly filter equipment. Tolerating noise is of course especially easy if your aquarium is located in a garage, basement or similar where you do not sleep. If you are on a very limited budget, you may be able to counteract problems that arise from inefficient filtration by spending more time caring for the aquarium.

  • Perform small and very frequent water changes and carefully monitor the water quality.
  • Remove free-floating debris by hand.
  • Remove any leftover food directly after each feeding session.
  • Avoid keeping messy eaters.
  • Check the aquarium several times a day in order to instantly sweep out dead or seriously ill fish that may otherwise pollute the water.
  • Include a lot of live plants in the setup.
  • Introduce scavengers.
  • Before you stock a new aquarium, you should wait until a really large colony of beneficial bacteria has established themselves since they will provide biological filtration.

Top Aquarium Filtration Methods

1. Mechanical filtration

Mechanical filtration removes larger debris from the water. A typical mechanical aquarium filter includes a container, a pump and some type of filter media. The pump will push water through the filter media and debris will get caught. By doing this, the mechanical filter can simultaneously aerate the aquarium water.

There are many different types of mechanical filters on the market. They can be classified into two broad categories: internal filters and external filters. Just as the name suggests, an internal filter is put inside the fish tank (e.g. the common corner filter) while an external filter is placed outside the aquarium (e.g. a canister filter that is hung outside the fish tank or placed on the floor under the aquarium).

Before picking a mechanical filter, it is important to find out the amount of water it will circulate per hour. Roughly speaking, a mechanical filter should have the capacity to circulate the water in the aquarium at least twice an hour. The exact recommendations will, of course, vary depending on an aquarium set up.

The coarseness of the filter media is naturally of imperative importance. A fine filter medium will catch a lot of debris, but will also be prone to clogging and you will have to clean it very often. A coarse filter medium can be cleaned less frequently, but it will be less efficient when it comes to actually catch debris floating around in your aquarium. Many aquarists combine fine and coarse filter media, e.g. by placing a coarse filter before the fine filter.

2. Biological filtration

Biological filtration relies on two types of beneficial bacteria. Ammonia is produced by fish and other animals in your aquarium, and high ammonia levels are unhealthy for the aquarium inhabitants. The first type of beneficial bacteria consumes ammonia and produce nitrite. Unfortunately, nitrite is even more dangerous than ammonia. The good thing is that the second type of beneficial bacteria will consume nitrite and excrete nitrate, a much less dangerous compound. The nitrate will be removed when you perform water changes.

To promote high amounts of beneficial bacteria in your aquarium, you can give them suitable homes. The filter media in a mechanical filter is, for instance, a great home for beneficial bacteria. Beneficial bacteria will also colonize sand, gravel and plant leaves.

You can kick-start the bacterial colonies in your new aquarium by purchasing a bacterial solution from the fish store. You can also use filter media or gravel from an established aquarium, but be careful as this can introduce malevolent organisms too.

3. Chemical filtration

Activated carbon is the most popular kind of chemical filtration. Mechanical filters are sometimes sold with a piece of activated carbon that will perform chemical filtration. Activated carbon is able to bind a rich assortment of different compounds, hence removing them from the water. When you have used powerful chemicals to treat infections in the aquarium, it can be a good idea to use some activated carbon to bind any leftover medication. It is quite common for keepers of densely planted aquariums to avoid using activated carbon since the carbon might tie up compounds that the plants need and thereby cause malnutrition.

4. Corner filter

Corner filters cost less and are easier to install. When you set up your first aquarium, chances are really high the pet shop will recommend you buying a corner filter. A basic corner filter consists of a plastic container and some form of the filter medium. The filter medium can, for instance, be sponge, foam or floss. The filter will also have a pump that constantly forces water into the filter, through the filter medium, and out again. A corner filter will therefore not only filter the water, but it will also aerate it.

The filter medium will of course act as a mechanical filter by trapping larger debris, but if you take good care of your corner filter it will eventually start performing biological filtration as well. Biological filtration is conducted by beneficial bacteria and the filter medium is an ideal home for such bacteria. Unfortunately, many beginner aquarists unknowingly kill off the beneficial bacteria. Filter sponge, foam or floss must be cleaned regularly to remove all the stuff trapped inside it and prevent clogging. The cleaning must, however, be gentle; otherwise, you will kill the beneficial bacteria. Never use any type of detergent and avoid using water hotter than the aquarium water. If you need to replace your old filter media, save some of it and add it together with the new filter media. This way, beneficial bacteria living in the old medium can move over to the new medium and colonize it.

A problem with corner filters is that they are not that effective. For aquariums with delicate species, more powerful filters or a combination of several different filters is usually required. If you have messy eaters in your aquarium, a corner filter will get clogged constantly and need a lot of work from you. Another problem with corner filters is that they can produce a lot of noise. The exact amount of noise varies from model to model. Corner filters can also look ugly and be hard to hide in the aquarium.

5. Canister filter

Canister filters are often found in really big aquariums or small aquariums with messy eaters. A canister filter is hung outside the fish tank or stored away somewhere else near the aquarium, e.g. under the aquarium inside the aquarium furniture. There is also a special type of canister filters known as submersibles that can be placed inside the fish tank. Submersible canister filters are however quite rare as most aquarists like the idea of being able to hide their canister filter away behind or under the aquarium.

Inside a canister filter, you will find glass wool, micron filter cartridge or a similar type of filter media. Just as in the common corner filter, a pump will push water through the filter media and larger debris will get caught. It must be cleaned regularly to prevent clogging.

Since canister filters are much larger than corner filters, they are capable of performing more powerful filtration. Their size can of course simultaneously be a problem for the aquarist, especially when there is no suitable aquarium furniture to hide the ugly filter from sight. Before you hide your canister filter, make sure that you must be able to access it regularly to clean it.

6. Undergravel filter

Inside an undergravel filter, powerful biological filtration can take place. The undergravel filter can also serve to trap larger debris and prevent them from floating around in the aquarium. An undergravel filter can, however, clog up quite easily and such clogging must be counteracted immediately.

The basic idea of the undergravel filter is to push water through a thick layer of aquarium substrate by using airlifts or powerheads connected to airlifts. The construction will usually involve placing the gravel on top of perforated plates. Beneficial bacteria will colonize the substrate and when water filled with organic waste is forced through the substrate, it will get caught by the bacteria and transformed into less dangerous compounds.

Bacteria will live on the surface of gravel and sand and the size of the substrate is therefore important. The smaller the gravel size, the more surface area per cubic inch. Small gravel will, therefore, have more room for bacteria than large gravel. The recommended gravel size does, however, vary greatly depending on whose opinion you seek. If you ask three different aquarists about which type of substrate to use for your undergravel filter, you may very well end up with three completely different answers.

As mentioned above, undergravel filters clog quite easily and they can be a bit tricky to clean. To prevent clogging, you can place a sponge at the water intake. You must of course regularly clean out the sponge if it becomes filled with large debris. Advanced aquarists sometimes counteract the clogging problem by using reverse-flow undergravel filtration, i.e. using a powerhead to force water up through the gravel. Really maxing out your powerheads to create super fast water circulation in the fish tank is not a good idea, since this will counteract the main idea of an undergravel filter.

Undergravel filters are inexpensive and quite common in freshwater aquariums. They work in saltwater as well but are not very popular since they are not efficient enough for sensitive marine species.

7. Fluidized bed filter

Unlike many other popular filters, a fluidized bed filter will not force carbon dioxide out of the water. Fluidized bed filters are therefore especially popular among those who wish to keep planted aquariums.

A fluidized bed filter is quite similar to an undergravel filter but consists of sand particles. A fluidized bed filter has smaller particles, is really compact and provides more powerful biological filtration. It will also require a more potent water flow. In a fluidized bed filter, the sand will be tumbled around constantly and the water gets really aerated. Clogging is less common in fluidized bed filters due to this violent tumbling.

There are several different types of fluidized bed filters to choose among in the aquarium hobby. If you are a handy person, you can even make your own. One type of fluidized bed filter is hung outside the aquarium, while others should be placed inside the fish tank. Do not skimp on pumping capacity when you get a fluidized bed filter, strong water circulation is really important.

8. Protein skimmer

Also known as foam fractionators, the protein skimmer is a comparatively new invention and it plays an important for all those interested in keeping saltwater aquariums. Marine flora and fauna tend to be very sensitive to high amounts of organic waste in the water and keeping them alive in aquariums is therefore difficult without powerful filtration. A protein skimmer will aid you by removing organic waste products from the water before decomposition sets in.

A protein skimmer has a tall tube in which air bubbles are formed that attract organic waste as they rise up through the water column. Organic compounds stick to the surface of bubbles and travel with them up to the surface. As the air bubble burst at the surface, the organic waste falls of and is collected in a cup. As mentioned above, one of the great advantages of the protein skimmer is that it will eliminate organic waste from your aquarium before it starts to decompose and foul the water.

If you have a freshwater aquarium, the protein skimmer will be of no use since it requires saltwater and a high pH value to work properly. For those with saltwater aquariums, a protein skimmer is really useful and few saltwater aquarists refrain from using them today. If you want a really delicate saltwater setup, such as a reef aquarium with corals and anemones, a protein skimmer is virtually mandatory.

9. Wet/Dry filter

The wet/dry filter is a type of biological filter. By installing a wet/dry filter, you create an ideal home for aerobe bacteria. Aerobe bacteria must have oxygen to survive and a wet/dry filter forms an oxygen-rich environment in which they can thrive. Wet/dry filters exist in many different forms and designs, but all of them are based on this principle.

Water is pumped into the dry/wet filter from the aquarium and a rotating spray arm or a drip plate is then used o disperse the water. The water is trickled over a material known to be appreciated by aerobe bacteria. This material is kept in a wet/dry filter chamber. The aerobe bacteria inside the wet/dry filter will have constant access to oxygenated water rich in organic waste, which means that they can thrive and multiply rapidly. Bacteria will convert harmful compounds into less harmful compounds, and the water will then be pumped back into the aquarium.

If you get a wet/dry filter, you must guard it against clogging. Clogging is one of the most common problems in wet/dry filters since large debris from the aquarium easily blocks the filter material. Many aquarists solve this problem by filtering the water at least once before it reaches the wet/dry filter. You can, for instance, install a micron filter, a sponge filter or use some filter floss to prevent larger debris from ever reaching the wet/dry filter. In saltwater aquariums, the wet/dry filter is often placed after the protein skimmer.

Since water is trickled in a wet/dry filter, they are sometimes referred to as trickle filters. You can also find them under the name bio-towers in aquarium shops. The wet/dry filter works in a freshwater aquarium as well as in a saltwater aquarium, but it is most common in the latter one due to marine creatures’ sensitivity to organic waste and demand for powerful filtration.

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