Internal filters for aquariums

Hello reader, Welcome to the special article on internal filters for aquariums!

Cheap and easy to maintain
Simple to install and use
Versatile and quiet

An internal fish tank filter is a submersible filtration device that is placed inside the aquarium itself.

Unlike external filters, which must be kept in cabinets or hung on the back of the tank, internal aquarium filters are placed inside the tank itself. This determines certain characteristics that provide a series of advantages and disadvantages, which we will see later in this article.

The submersible fish tank filter is ideal for small fish tanks, but can also help in large tanks. They can be the solution with dead zones, or can also be used to provide additional filtering.

These internal filtration devices are small, quiet and very easy to install. They provide excellent filtration and are quite inexpensive. In this article we are going to tell you everything about this type of filter, and in addition, we have these specials on the best internal filters:

Comparative table of the best internal filters

Internal aquarium filters are the most popular type of filtration system on the market. It has the widest range of models among all types of filters. You just have to look at the Amazon storefront to see the wide variety of internal filters for aquariums that there are.

Thanks to the great versatility of the internal aquarium filter, they are used by all kinds of aquarium hobbyists. From beginning hobbyists to expert aquarists, owners of nano aquariums, quarantine tanks, large cichlid aquariums or even “weird bugs”.

What is the best aquarium internal filter?

An experienced aquarist knows that there is no best internal aquarium filter, just as there is no best aquarium light. There is no perfect inner filter for all situations.

It depends on the type of tank, its characteristics, conditions and circumstances. A certain filtering system may be the perfect solution in a certain situation, but it may be useless in other circumstances.

Inner filters are quiet, inexpensive, and great for a small size tank. They are very common to use among beginners.

To buy an internal filter for a fish tank, it is recommended to study the characteristics of the tank, and to know a little about the filters to know which is the best internal aquarium filter.

At we like to make things easier for you, and that is why we are going to review several of the best internal aquarium filter models.

HAGEN Fluval U

Fluval U Series presents four models of different power for small and medium aquariums up to 240 liters. Click on the button below the image to see the 4 models of this filtering system.

These internal aquarium filters have three outlets. Offers water flow from top outlet, bottom outlet, or built-in spray bar mounted vertically in the middle.

The flow from the top and bottom outlet is directionally controllable, and is supplied with a Venturi.

The device is supplied with filter media. These inner fish tank filters come with two sponges for alternative cleaning. Also a polyester-carbon cartridge for additional mechanical and chemical filtration, and a Biomax bioloader.

The Fluval U1 model includes only one sponge due to its small size.

It has a hinged lid that makes it easy to access the filter media for replacement or cleaning. It also features vertically mounted media trays that easily slide in and out of media trays.

The entire filter system can be released from its mounting bracket to be removed from the aquarium.

If you want to delve into the characteristics of this internal filter from the German company Hagen, you can see our review of the Fluval U internal filter. We see all its features, technical specifications, cleaning tips and user opinions.

Eheim PickUp

The Eheim PickUp internal filter is a series made up of four models of compact filters for small and medium-sized tanks. Press the button under the image to see this series of internal filters in the Amazon online store. It comes with the necessary filter charges, ready to use and is ideal for beginners.

The main concept of this internal aquarium filter is the «pickup», which means to collect. It refers to the ability to “pick up” the container with the filter loads, leaving the fixing system in place. This greatly facilitates maintenance.

The outlet mouth is rotatable, so the outlet flow can be directed. It also comes with a diffuser to achieve optimal oxygenation of the water, but only in the top three models.

This series of internal aquarium filters also includes a water flow regulator to be more practical and versatile. In our review article about the Eheim PickUp you have more information about its features and user opinions.

Tetra IN Plus

This series of In Plus filters from the Tetra house has five models that adapt to small and medium tanks up to 200 liters. Press the button under the image to see the offer of this range of filters on Amazon.

The Tetra In Plus internal aquarium filter also comes with the necessary filter media. And it has a practical device to remove the filter material in a simple way.

It has two sponges in two different chambers, to be able to carry out cleaning and maintenance alternately.

This filtration system features a 180° rotating water outlet and a stepless flow regulator for a perfect fit. It also has an adjustable venturi system that makes an extra contribution of oxygenation to the water.

The compact and elegant design of this device does not require much space, being easy to hide. The guarantee offered by the house is two years, it is approved by the TÜV/GS2. In our review article of the Tetra IN Plus you can see in more detail the characteristics and opinions of the users.

Penn-Plax Cascade

The Penn Plax Cascade Internal Filter is a low cost, heavy duty filter with surprisingly good reliability. Click on the button under the image to see this internal filter.

The water circulates through four stages of filtration, leaving the water crystal clear, while increasing the colony of beneficial bacteria. If you are looking for more than just an internal filter without spending a lot of money, this may be your choice.

The Penn Plax submersible aquarium filter comes in multiple models. The first model in the series is the 175, designed for nanoaquariums and terrariums up to 20 litres.

The larger Cascade 300, Cascade 400 and Cascade 600 filters provide biological, chemical and physical filtration, making them the ideal choice for aquariums up to 180 liters.

It has a spray bar or outlet flute that distributes the water evenly over the surface of the water to promote adequate oxygenation of the aquarium and achieve a healthier and cleaner environment.

The internal sponge optimizes the colonization of beneficial aerobic bacteria for safe and healthy aquarium water.

An activated carbon cartridge is included for efficient chemical filtration of harmful chemicals, toxins, odors, discoloration, and other contaminants. The carbon cartridge is easy to replace for easy maintenance.

The container of this internal filter has a large capacity, allowing you to choose your own filter medium.

The pump features adjustable and directional water flow. The filter can be placed vertically or horizontally. If you are interested in this filter, you can see our review of the Penn Plax Cascade, where we take a closer look at this internal filter.

Internal filters for aquariums

Any filtration system that is located inside the tank itself is an internal aquarium filter. These internal fish tank filters are the easiest way to attach a filtration device to your aquarium. This is why they are very typical of kits.

The internal aquarium filter provides a way to sanitize and purify the water from inside your aquarium. They are compact, simple to install and easy to maintain.

In addition, they are usually extremely quiet, since the motor is under water.

Instead of hanging off the back of your tank like the waterfall fish tank filter does. Or hide in a closet, like the basket strainer. The aquarium internal filter is placed inside your tank, and is held by suction cups to the glass.

If you have a small tank, submersible fish tank filters are great. Also if you want to fit your tank against the wall, or if you need a little extra filtration in a larger tank.

Unlike external filters, the aquarium internal filter does not take up additional space.

The small size allows the fish tank internal filter to be easily concealed behind aquarium decorations. Using aquarium plants or decorative rocks, it hides quite well, making it the perfect filtration solution for many types of tanks.

At we want to present you the best internal aquarium filters. High quality internal aquarium filters from the most trusted brands in aquarium filtration: Tetra, Eheim, Fluval

About the internal filter for aquarium

As we have already said, an internal filter is generally used in aquariums of a small or medium size. But the internal fish tank filter is prepared to deal with a wide variety of situations.

If you are new to this beautiful hobby of aquarism, I recommend that you take a look at our guide to aquariums for beginners.

In addition to being a primary filtration system, the internal fish tank filter is also very useful as a back-up filtration. Sometimes it happens that a medium or large tank has some area that the current filter does not reach, known as «dead spot».

A small fish tank filter is a great way to bring water circulation to that stagnant area.

The negative part of this type of internal fish tank filter is that it takes up space inside the aquarium, and the filtering capacity is usually quite limited. In addition, it is not usually very aesthetic within the frame of the fish tank.

In the antipodes of the internal filter is the external one, and in that link you can see our special on this type of filters external to the tank. We also have an article about the best canister filters on the market.

How does an internal aquarium filter work?

The internal aquarium filter is usually a rectangular box placed with suction cups on the glass of the tank. Click on this link to see that it is the predominant form, but not the only one. They can have two or three sections inside, to divide the filter materials, the pump and sometimes also a water heater.

Like any filtering system for aquariums, the internal filter for fish tanks is based on the nitrogen cycle. You can see our article on the cycling of an aquarium what is this cycle of nitrogen and nitrifying bacteria.

We can see in the image that the operation of an internal filter is very simple. The filter has an inlet mouth, usually at the bottom, through which the pump draws dirty water from the aquarium.

Inside the device, the water passes through the filtering materials that are responsible for purifying the water. Finally, the clean water is returned to the aquarium.

With this simple scheme, we have many different designs, with different configurations in their filter loads.

Internal filter loads

Regarding the filter material, the most common is that the fish tank filter comes prepared to work with the necessary filter material. Make sure the filter media is correct for your internal aquarium filter and that it is properly placed inside it.

Due to its limited size, the most common is that the filter comes with a sponge that performs the mechanical and biological filtering work.

The filter media used in the aquarium is a personal choice, especially for experienced hobbyists. But it is always advisable to use two different filter media, one mechanical and one biological. This will greatly help its efficiency, and when it comes to maintaining the internal filtering system for fish tanks, as we will see later.

  • Place a bit of sponge as the first filter load to block the passage of leaves, food scraps and other solid waste.
  • Then some biological material like ceramic rings. Most of the colony of nitrifying bacteria that purifies the water settles in this medium.

If your internal filter allows you to add more filter media, add another sponge after the bioload, before the pump. Thus, you minimize the possibility of debris entering the pump of the internal filtration system for fish tanks.

Let us now see how to install a fish tank filter.

How to install an internal fish tank filter?

To the question about where to place an internal aquarium filter, they should be placed at the bottom of the aquarium. This is done to prevent debris from building up in the back of the tank.

Regarding the question of how to place an internal aquarium filter, it should be placed with the clean water outlet, causing the movement of the surface of the aquarium water.

The movement of the water surface generates gas exchange, favoring a higher concentration of oxygen in the tank water.

If possible, it is also advisable to find a place in your fish tank that allows you to easily hide the filter. Placing the internal aquarium filter in a back corner is usually one of the best options.

How to clean an internal aquarium filter

Cleaning some internal filters can be difficult and messy, and they also return some of the dirt to the aquarium. Others have separate basket sections that are easier to remove without dirtying the water.

The first thing to keep in mind when cleaning the internal fish tank filter is that we have to preserve most of the bacteria colony.

When the filter has only one sponge that does all the work, it should ALWAYS be washed with aquarium water. Do it gently, since the colony of nitrifying bacteria is there, and we don’t want to lose it all.

If your filter has two sponges, then you can wash them without fear alternatively. The biological filter should not be cleaned so often, because most of the nitrifying bacteria are there.

If you wonder how often you should clean the filter, who will answer your own aquarium. When you notice that the flow rate of the filter has been noticeably reduced, it is time to clean.

Main characteristics of an internal filter

Let’s see the main characteristics that any internal fish tank filter should have.

adjustable flow

Depending on the aquatic beings in your aquarium you will need a slower or faster flow. Therefore, simple and effective flow control is expected.

Directional flow control

If your internal aquarium filter is located in a back corner, you may not want unidirectional water flow. It gives a lot of flexibility to have the option to direct the flow to the left or right, and up or down.

Venturi Option

The Venturi effect uses the flow of water from the filter outlet to absorb atmospheric air and generate bubbles. This free form of additional aeration benefits cold water aquariums and highly populated aquariums.

Remember that the oxygenating function of these bubbles is carried out when they reach the surface of the water and shake it. The bubbles themselves rising through the water do not oxygenate the water.

Basically this benefits any aquarium that contains fish, but not where plants or corals predominate. The air bubbles irritate the corals and cause them to expel CO 2.

Normally in the internal filter for fish tank you can regulate the amount of air that is produced with the venturi.

Filter Media Options

The ability to hold additional media is a feature that has become very important in recent years. The more filter loadings the better, as it means more cleaning power and more debris reduction power.

For most aquarists, the best option is the use of extra biological media. Faced with a filter material that performs mechanical and biological filtration, it is better to have two materials that do the job separately.

For most aquarists, an extra bioload means a better internal aquarium filter.

double sponges

Keeping with the theme of filter loads, having two sponges is very useful and practical for alternative cleaning.

If the sponge is the only media in the internal filter, it has to perform mechanical and biological filtering. So the sponge will need very frequent cleaning, but the problem is that cleaning puts the bacteria colony at risk.

The solution to this problem is two sponges so that they can be cleaned alternately. Even though they alternately replace themselves, the colony of bacteria remains almost intact.

Pros and cons of the internal aquarium filter

Advantages of internal aquarium filters

  • They are cheap filters.
  • The internal filter for fish tank is ideal for small aquariums with a small number of fish.
  • They are very easy to install and maintain.
  • Most are very quiet devices.
  • These filters can have different applications:
    1. Extra filter device.
    2. Some can be placed horizontally or vertically.
    3. They activate ultraviolet sterilizers.
    4. Creation of additional currents.
    5. They are the only solution when external filters cannot be used.

Disadvantages of the internal filter

  • The filtering capacity that an internal aquarium filter usually has is usually quite low, which is why they are used for small or support tanks.
  • They also take up space inside the tank. It is a space that removes plants, fish… reducing the capacity of the tank.
  • In addition, as happens with backpack filters, this filtration device is visible, within the set that makes up the fish tank.

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