All you need to know about an Aquarium Filter

aquarium filter

If you want to set up an aquarium, you also have to deal a little with the technology required for this. An aquarium filter is an integral part of the aquarium technology.

What does an aquarium filter do?

In addition to the lighting and possibly necessary heating, the filter in the aquarium plays a very special role. The Aquarium filter has 2 important functions:

  1. Prevents too much waste from accumulating in an aquarium
  2. Ensure that a biological climate can be created in which plants and animals feel comfortable.

An aquarium is an artificial and closed biological system. Therefore, natural pollution cannot easily be broken down or otherwise removed.

In this article, we will go over the following key components that relate to aquarium filters:

  • What a filter can do in an aquarium
  • Which types of filters are widely used in aquaristics
  • What should be done when cleaning the filter

Just like all other technical components of an aquarium, the filter functionality should be checked regularly. Should a filter malfunction go unchecked, the consequences for the biological system and thus also for the aquarium inhabitants would be dire.

Water purification through the Aquarium filter

Contamination occurs in every aquarium – this is also due to the aquarium inhabitants and the plants, which otherwise also provide very important services for the biological balance in an aquarium.

The core functions of an aquarium filter

1.      Filter out impurities

The basic task of a filter is to filter out impurities that are in the aquarium water.

Depending on the type of filter, the water is slowly sucked in from the aquarium, passed through the filter material and returned to the aquarium when it has been cleaned.

Although there are different types of filters that function slightly differently, their ultimate goal is to remove contaminants from the aquarium water.

2.      Trap important bacteria

The filter is also an ideal place in the aquarium where important bacteria can settle.

A lot of beneficial bacteria develop within the biological section of the filter during the filtration process. This then breaks down harmful fish waste. These bacteria actually help to keep fish alive, by first converting ammonia, then nitrite to nitrate. Below is a summary of the importance of bacteria in the filter.

Bacteria in the filter and their development

Beneficial bacteria are essential to fish keeping and the aquarium as they process the toxic biological waste produced by the fish. The most toxic substance is NH3, free ammonia and this comes from the fish passing urine and excreta from the gills.

Types of beneficial bacteria in an aquarium

An aquarium filter accumulates two main types of beneficial bacteria required for water purification. In older aquariums they live everywhere, but most prolifically in the filter system where conditions are ideal for them.

1.      Nitrosomonas

These are aerobic bacteria which convert deadly ammonia into another less toxic substance called nitrite.

2.      Nitrobacter

These are aerobic bacteria, which converts nitrites into less harmful nitrate. Without the bacteria, toxic pollution can quickly build up and kill any fish present.

Bacteria and new aquariums

New aquariums often suffer from what is commonly referred to as “new tank syndrome”. The best way to avoid this is by slowly cycling or maturing the aquarium.

This is essential because there will be very few bacteria present in a clean new aquarium/filter. Bacteria need time for multiplication in order for them to be trapped in the aquarium filter and their benefits to be achieved.

How to quickly mature the aquarium

  1. Start by slowly adding fish stocks gradually and feeding lightly.
  • Continuously monitor the NH3/NH4 level to ensure the fish are safe. If ammonia rises, you can do things about it.
  • To accelerate the growth of the bacterial colonies, migrate bacterial colonies from a disease-free established aquarium by moving gravel, décor or filter media to the new tank.
  • Add some dirty water from your local fish store to help kick-start the biological processes.


  • Bacteria are a living organism. They therefore need to be cared for just as anything else does. Large swings in temperature, pH and pollution can cause bacteria colonies to reduce or die away, even in mature aquariums.  
  • Although fishless cycling to establish bacteria before adding fish are sometimes used, they do have  some risks, such as understanding where the ammonia level is at all times with exhaustive testing regimes.

What types of filters are there?

There are many different types of filters, all of which have the same task, namely to reliably filter the water in the aquarium.

2 Factors to consider when choosing an aquarium filter

  1. It is important that you buy the right size of the filter. So opt for a larger model, which may clean more water than a filter that is too small, which cannot cope with the amount of water in your aquarium.
  2. It is important that you are able to maintain all the bits of the filter so that it have a long service life and always reliably keep your aquarium water clean.

Types of aquarium filters

The mechanical filter

aquarium filter

A mechanical filter filters the coarse and fine dirt from the aquarium water. It is suitable both as a pre-filter and as a stand-alone filter system.

The individual models convince through a simple change of the filter material and are easy to attach and remove again if necessary.

While this filter should have a minimum flow rate of two to four times the water volume in freshwater pools, it must be at least 10 times the volume in seawater pools. For this reason, many aquarists change the filter substrate every week, but this means that the mechanical filter can never function as a biological filter with many important bacteria because these are destroyed during cleaning.

Internal motor filters, for example, which are available in numerous designs, are particularly ideal as mechanical filters.

Trickle filter

aquarium filter

Trickle filters are rarely used. These work as so-called “super aerobes”.

The water is applied to the filter material, which naturally has air contact and is then fed into a separate basin. The water is then pumped back from this basin.

NOTE: Trickle filters only work effectively when at least 4,000 liters of water run over the filter material per hour, which is rarely the case.

Anaerobic filters

aquarium filter

An anaerobic filter is a good method of biological filtration. This filter works without oxygen.

With such a model, the filter material has to be flushed with low-oxygen water, which is only possible if the water flows slowly through it. If the water flows through very slowly, the oxygen is completely gone after only a few centimeters in the filter bed.

In contrast to other filter options, however, only nitrate is broken down, so that proteins and the like cannot be converted to nitrate and then broken down.

For this reason, these filters can only be used additionally and are unsuitable as sole filters.

Biological filter

aquarium filter

In these special filters, the bacteria in the filter clean the water.

Millions of small creatures, including bacteria, amoebas, ciliates and other animals, live in these filters and feed on the organic substances in the water.

The organic matter is removed or modified so that it can be added back to the water. These bacteria and other small living things can be recognized by the filter materials as brown sludge.

IMPORTANT: Do not wash them over and over again. They are good for the aquarium.  As long as there is still enough water flowing through the filter and it does not clog, everything is fine.

Proteins, fats and carbohydrates, which can all be found in aquarium water, are the main food for the microorganisms. These are converted into nitrate and carbon dioxide.

The biological filter is also suitable for all aquariums.

External filter

aquarium filter

This filter is located outside the aquarium and therefore does not interfere with the optics.

The water is transported through hoses, which are available with different diameters, to the filter, which is usually located in the aquarium cabinet. The water now runs through the filter, which can be filled with different filter materials, and is filtered there.

The filter material should also be selected individually according to the filling. After cleaning, the water is pumped back into the aquarium, which of course brings movement back into the tank.

ADVANTAGE: An external aquarium filter does not take up any space in the aquarium and does not impair the optical image.

Internal filter

aquarium filter

In contrast to the external filters, these are of course internal filters. These suck in the water, clean it inside using individually selected filter material and then return the purified water.

Advantages of internal aquarium filters
  • No hoses are required
  • They can be ideally used as flow generators
  • They are available in numerous sizes

While some models can be used as pure aerobic filters, there are also models that filter part of the water anaerobically and the other half aerobically.

Disadvantages of internal aquarium filters
  • Internal aquarium filters take up space
  • They have to be completely removed from the basin for cleaning each time

How to choose the right filter for your aquarium

This video discusses how to choose the right filter type based on your needs. It takes a look at the many popular filtration methods such as, HOB, sponge filters, canister filters, matten filters, sumps and more!