Best External Filters For Sale-Buying Guide

What is an external aquarium filter?

An external aquarium filter is an accessory used to recirculate the aquarium water through a series of filter materials, in order to remove impurities from the water, and then return the clean water back to the aquarium. They are the mechanical part of the aquarium filter system.

It is called an external filter, because the filter is placed “outside” the aquarium, instead of inside the aquarium (internal filters) or on the side of the aquarium (backpack filters).

They are also often referred to as aquarium basket filters or canister filters, because they have a certain similarity (physical, not real) to the filters of the same name used in petrol engines.

How do external filters work?

The operation is relatively simple:

  • A pump draws the water from inside the aquarium, using a hose. This is called the outlet.
  • Once the water has been removed, it flows through the rubber to the filter. The pump itself circulates the water through various filter materials (stages), with the intention of cleaning the water of its impurities.
  • After passing through the different filter materials, the clean water is returned to the aquarium through the return outlet.

The different stages of filtration that take place in an external filter are:

  • Mechanical filtration. The filter materials (sponges, foam), stop the larger impurities.
  • Biological filtration. Within the filter materials (porous canutillos or similar), there are some bacteria that are responsible for reducing toxic biological waste from the aquarium, to convert them into harmless matter. The biological filtering materials, favor the appearance of these bacteria.
  • Chemical filtration. The water finally passes through filter media (active carbon), which removes chemical residues in the aquarium water, such as drug residues.

How often do you have to clean the external filter?

One of the great advantages of external filters for large aquariums is the low maintenance required. According to aquarium enthusiasts, the cleaning of the external filter does not have to be routine, every three months or six months, but rather when there is a decrease in the flow rate. Obviously, the maintenance of the aquarium external filter is quite personal, but what is clear is that it does not need to be cleaned frequently.

Precautions or tips for cleaning an external filter

  • If we are going to replace some of the filter materials, never replace more than one at a time, to avoid the loss of bacterial colonies.
  • To clean the foamex, always do it with water from the aquarium, to avoid the same: loss of bacterial colonies.
  • The filtering tubes do not need to be changed, they last for years. If we see them too dirty and want to clean them, we can do the same operation, always with water from the aquarium. Just rinse them.
  • It is advisable to strengthen the bacteria population on a regular basis. Every time we clean, add new bacteria with one of the products recommended for this purpose.

Which is better, internal or external aquarium filter?

All aquarium filters are good, as long as they do their job effectively.
The big difference between internal and external aquarium filters not only lies in their installation inside or outside the aquarium, but also in their efficiency, ease of handling and cleaning.
It is obvious that in this uneven battle it is always best to use an external filter, even for a small aquarium.
External fish tank filters are much more efficient, for the simple reason that they have enough space to incorporate a larger amount of filter material inside.
They are also much easier to handle and clean, as they are outside the aquarium.
Another big difference to internal filters is that they do not take up any space inside the aquarium.

Reducing the flow rate of the aquarium filter

  • An option available on the external filters, and almost never available on the internal filters, is the possibility to reduce the filter flow rate.
  • The filter current in the aquarium is sometimes too much for the fish, which need calmer water. External filters have a regulator, which allows us to reduce and adjust the flow according to the population of the aquarium, as simple as turning a key.

Advantages of external aquarium filters compared to other types of filters

There are some technical features that make external filters one of the best possible options for maintaining water quality in the aquarium:

They provide a much higher filtration

To maintain optimal water quality parameters, a filter must be capable of recirculating the aquarium water a minimum of 3 times the total volume in one hour.
This is known as water flow, which in external filters is adjustable, in case there is a need to increase or decrease the flow.
As an example, a 100-liter aquarium should have a filter with a water flow of 300 liters/hour. This is a minimum measure, because if your aquarium has a very dense population of fish, you should have a filter with a higher performance.
It should also be taken into account that in many cases, the water flows referred to by the manufacturers are theoretical and not real. By “filtering” the water, making it recirculate through the different filter materials, the flow capacity of the filter is usually reduced by 35%.
These and other details, such as the amount of real liters that your aquarium has, will help you decide which filter to buy.

More efficient filtering capacity

It is clear that its filtering capacity is much higher than that of internal filters or waterfall filters.
While the former, due to their size, are not capable of storing many liters of water inside, the external filters can retain up to one liter of water.
This fact may seem unimportant, but it is not.
Imagine a backpack filter that is capable of filtering 600 liters per hour, with a capacity to retain water inside 200 ml, while an external filter of the same filtration conditions, retains 1000 ml.
The first one has to make the water recirculate at full speed, while the second one does it at a lower speed.
In practice, the filter that retains more water inside, is also capable of retaining a greater amount of particles, resulting in a much more efficient filtration.

Maintenance and cleaning tasks are reduced

The maintenance tasks of the filters are necessary and recurrent. From time to time we must clean the filtering materials, removing the remains they remove from the water.
These tasks are much easier to carry out on the external filters than on the internal ones, as we do not have to put our hands into the aquarium.
The size of the external filters also justifies that they do not need as much attention as the internal filters, some can be cleaned every three or six months without any problem.
If you are wondering how to clean an external aquarium filter, you should know that they can be easily dismantled, the access to all materials is much easier than in other filters.

Most customizable filter media configuration

External filters have much more space inside, so they leave us more freedom to configure the filtering materials that will be part of the filtration.
In the internal or backpack filters, it is very common that the brands have their own filtering cartridges, without much freedom of configuration.
They can hold up to five filtering stages, two mechanical, one chemical and two biological, achieving a much more complete filtration than any other type of filter.

They are very quiet

The external filters are much quieter than the backpack filters, although it is difficult to compete with the internal ones, which produce the filtration inside the aquarium and therefore the noise is totally hidden.
If we analyze the efficiency of the filters and compare it with the noise, without any doubt, the external filters are the best option.

Aesthetic and space improvements: Where and how to place an external aquarium filter

You will find that there are all kinds of external aquarium filters. Some, like the Eden 500 series, can be placed right next to the aquarium, if you have little space, you don’t want to or can’t hide the filter.
Most of them are designed to be away from the aquarium, usually in a table under the aquarium, so that noise is reduced, aesthetics are improved (there are no filters in sight) and space is gained inside the aquarium.

Buying an external filter How do I choose from so many?

Choosing is always difficult. If we had two options it would really be…A or B, but “fortunately” this is not the case, there is a great variety of aquarium filters, of different brands, with sizes and prices that fit all needs.
Even so, I wanted to highlight what are the main characteristics that you should analyze from a filter, to determine if it fits your aquarium:

Flow rate: volume of liters filtered per hour

To determine whether a filter is suitable for a particular aquarium, we need to know its flow rate or volume of liters filtered per hour.
Manufacturers always give the wrong figures, so ideally we should buy a filter with a higher flow rate than we need.
And which one do we need? Well, it is logical and recommended that the chosen filter is capable of moving at least three times the total volume of the aquarium.
If you have a 100-litre aquarium, it is advisable to buy an external aquarium filter with a flow rate of 300 liters per hour.

Capacity in liters of filtering material

There are many ways to measure the efficiency of an external filter, one of them is the amount or volume of filtering material it can hold inside.
In the end, the filter is simply a pump that takes the water out of the aquarium, recirculates it through a filter material, and then flushes it back cleanly into the aquarium.
The size of the filter, the amount of filter material and the volume in liters of the filter cup must be analysed.
The reason for this is very simple: if the glass is too small for the amount of flow (litres/hour) it is able to filter, this means that the water passes through the filter at full capacity, without the filter media having a chance to do its job: filtering.

Filter media and multi-stage filters

eheim experience filtersOne of the advantages of external filters over internal filters or backpack filters is that the content of filter materials is easier to modify to our taste.
In aquatics, as in almost everything in this life, everyone has their own tricks for improving the aquarium water parameters, and here the filter has its importance.
All kinds of materials can be added, such as peat, pieces of shells…to raise or lower the acidity, the pH…if the chosen filter has enough space, or better, is a multistage filter with different baskets for filter materials, these practices are easier to carry out.

Aquarium filtration needs, depending on fish and plants

A number of “basic” parameters or measurements have been propagated (I do this myself), to determine what is the minimum need for filtration in an aquarium, so that we can determine the size of the filter we should buy.
We must understand that these filtration needs: moving the volume of the aquarium, at least 3 times in an hour, are a reference.
If our aquarium is densely planted or the type of fish we have in the aquarium (goldfish, for example), have a high need for filtration, we should also take this into account when choosing the right filter.


Leading brands of aquarium filters are increasingly taking into account the efficiency and energy consumption of their devices.
Each filter has among its main characteristics, how much it spends.
To similar filters in working performance, we should choose the one that consumes less.
The filter is always plugged in and consuming energy. It may not be an excessive expense per month, but if we join all the months of the year, the “bill” changes.

Quality/price ratio

This is another point that we should consider before buying a filter, that’s why I have bothered to create a comparison table, because many times what is cheap, is expensive.
We can buy a cheap external aquarium filter, as long as the quality corresponds to the price.
We should know that the durability of a cheap external filter is probably less than that of a higher quality filter.
Choose well known brands, which have technical support. This is always a guarantee.

What is the best external aquarium filter on the market?

Pond filtersUse external aquarium filters, it offers countless advantages for your aquarium. Among the most important ones:

  • Cleaner water, thanks to the high levels of filtration
  • It is a perfect ally to keep the water parameters at the right levels
  • Life inside the aquarium, fish, plants…stays much healthier

There is no excuse for having an aquarium in perfect condition, there are external filters that take up very little space, that barely make any noise and that have an affordable price. It does not matter the size of your aquarium, they are filters for small fish tanks or for large aquariums.