Internal or external filters – where are the advantages?

Aquarium filter for indoor or outdoor?

In order not to anticipate all the fun, I will tell you at the end of the article which filter I like better. First of all, the comparison is a bit misleading – just like with apples and pears. So we compare them on the common denominator fruit, uh nonsense filter. It is true that every tank – no matter how small it is – needs a filter. This filter not only moves the water, but also filters dirt and harmful substances out of the water. Without a filter, the aquarium’s ecosystem would quickly change and die. So when setting up an aquarium, choosing the right filter is a crucial element.

How the aquarium filter works

The filter sucks the water on one side of the pool, filters it through various layers and releases it on the other side. Inside the filter there is a medium which has various functions. On the one hand, bacteria and small organisms live here, which get pollutants as food from the water. But also suspended matter and other impurities are taken out of the water. This second filtration is achieved by one or more media. Usually there are substrates and filter materials such as cotton wool, fleece, clay beads and stone or mineral substrates.

Among other things it is important how much water volume is filtered. Depending on the size of the pool you need a different volume. As a rule of thumb one can say that all water should be filtered through about twice per hour. This not only has the advantage of clean water, but also guarantees the redistribution and thus the temperature balance. Oxygen gets into the aquarium and plants get their nutrients. A light current is also created.

Filter media at a glance

Both filter types can be filled with various media. Depending on how many chambers or boxes the filter has, the substrates and media can also be combined. Here you can find the most common materials:

  • Ceramics
  • Sinter glass balls
  • Sintered glass rings
  • Carbon substrate
  • Filter sponge
  • Filter fleece
  • Cotton wool
  • Plastic balls
  • Peat
  • Coal
  • Zeolite
  • Coral Break
]All filter media are available in different sizes. They differ in pore size and substrate size. Each medium has its own effect, including the removal of nutrients, algae and pathogens.

Biological, chemical or mechanical

A sensible filter system not only ensures clear water, but also healthy water. A distinction is made between the following types of filtration:

  • biological
  • chemical
  • mechanical
]Mechanical filtration removes suspended solids. The biological filter provides the necessary habitat for useful microorganisms and bacteria. Chemical filtration makes the water clearer and possibly eliminates odours.

When is an internal filter useful?

Internal filters are generally used for small pools. Especially the beginner or aquarium novice often tends to use an internal filter. At first glance, the design seems simpler and also easier to maintain. This may be true, but with a little knowledge and experience, both the maintenance effort and the commissioning are kept within limits. You will have a lot of fun with only a few start-up difficulties afterwards. Actually, only the installation of the hoses is a little more complicated.

The decision for an internal filter is usually rather unconscious. For one thing, many choose a complete set. Here the internal filter is usually already included. On the other hand, the internal filter is much cheaper and with the price selection comes the choice for the right filter. An internal filter is not only less cost-intensive to purchase, but in most cases it also consumes less electricity. This is due to the technology. With an external filter, the water is moved over a longer distance, so the filter also has to be more powerful.

When do I need an external filter?

For larger aquariums you usually choose an external filter. It is the right choice from a water volume of about 100 litres. An external filter has a considerably stronger filter performance. Another nice detail is that you can make the external filter virtually invisible. With a base cabinet or a cabinet next to the aquarium it is practically stowed away and still accessible.

The advantage of the external filter is that it does not take up any space in the tank, so you can use the full tank. You can also mount additional filters such as a slow filter and UV sterilizer on the hoses without having to enter the aquarium.

The external filter in detail

As the name suggests, the external filter is not located in the tank, but outside the aquarium. The alternative are filters that are located inside the tank, such as the usual internal filter, the bottom filter or the Hamburg mat filter. However, the external filter cannot be placed in the place of any choice, but must be placed in close proximity. For the water to be exchanged correctly and for the filter to function properly, a place close by is absolutely essential. For this reason, most aquarium keepers store it in a cabinet.

The external filter also has a part of the filter system in the tank, including the inlet and outlet. A connection is made via green or transparent hoses. When the place for the aquarium filter is found, you can start filling it. Correctly heard, you have to insert the suitable medium beside the filter as described above.

External filters are usually also controllable, which means that you can regulate the flow rate. The external filter needs to be cleaned less often than the internal filter. Of course you should take care that the stocking density is not too high. By the way, there are also external filters with an integrated heater, then you save the heating rod.

Safety risk external filter

A central source of danger for your aquarium inhabitants is the intake manifold. Here a strong suction is created, which can suck in small animals and plants and thus become life threatening for them. Usually an attachment protects against this danger, which like a kind of sieve protects the fish and plants from being sucked into the safe suction - I mean death.

An additional aspect that should not be considered with the internal filter is water damage. If the filter suffers a defect or a hose comes loose, it can easily become annoying. Leaking water not only damages your equipment, but the decreasing water level is also dangerous for the inhabitants of the pool. Therefore, make sure to maintain the filter at regular intervals.

Cleaning tips

I experience aquarium owners again and again, who not only change the aquarium water much too often and in too large a quantity, but also keep the aquarium filter meticulously clean. This does not do the inhabitants and plants any favours. As described above, useful bacteria and living beings settle in the filter, which are essential for a healthy habitat. The filter is cleaned when the water cannot pass through and not when the medium is discoloured. When the time comes for cleaning, please do not overdo it, but rinse it gently with lukewarm water. Otherwise all bacteria populations will be removed. Never use any cleaning agents! Never change the water and clean the filter on the same day.

Lead time

If the filter is purchased new, give it a little time until it has collected enough bacteria. This means that you should only let the fish into the same tank after it has been running for a while. If the tank is not new and the fish are already part of the aquarium, then this is of course difficult. In this case it is advisable to let the old filter continue running. After some days you can take it out. So all beneficial insects can settle down in peace and one does not take any risk. Alternatively, you can put the filter medium into the new filter, if it fits.


The internal filter is used for smaller aquariums and is in most cases cheaper to purchase. The external filter not only has a higher capacity, but also offers more space in the aquarium for decorations, plants and animals. It is also much nicer in appearance because it can be placed out of sight and does not interfere with the aquarium design. The most important point is that the performance of the filter matches the size of the tank. Another nice thing is that with an external filter you can adjust the different media more precisely to your needs.

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