Fluval U Series – Review + Opinions

Hello refined reader, this time we are dealing with a magnificent internal filter for aquariums, the HAGEN Series U fluval filter.

Suitable for fresh and salt water
Adjustable flow control
Powerful water flow
Elegant design

HAGEN ‘s Fluval brand has become one of the best-known names among aquarium hobbyists.

For many years, it has provided one of the highest levels of quality, regardless of whether we are talking about an aquarium, a fish tank filter or any other accessory. This brand is highly valued by aquarists. Over time they have expanded their catalog, but their quality has not changed.

The Fluval U brand internal fish tank filter is the ideal filtering device, both as a single filter and as a backup filter. It is the perfect solution where there is no possibility of placing an external filter.

This HAGEN house filter has a 3-year guarantee, and purifies the water by performing a very effective biological filtration. It is a very efficient and silent device, as well as being very easy to use and maintain.

HAGEN has created the U-Series range of internal filters with 4 models that covers the spectrum of smaller aquariums and also slightly larger ones with low population levels.

  • Fluval U1 – Up to 45 liters.
  • Fluval U2 – Up to 110 liters.
  • Fluval U3 – Up to 150 liters.
  • Fluval U4 – Up to 240 litres.

The Fluval U underwater filtration system is divided into three stages for efficient filtration and maximum aquarium water quality. In this link you can see the offer of this range of filters on Amazon.

This internal filter is suitable for both freshwater aquariums and marine aquariums. It is also for reptile tanks.

Fluval U internal filter

The versatility of this device allows it to be used as a main filter or as a complementary filter. Thus, it can complement other filters to provide extra filtration and movement of water.

This internal filter has a powerful flow that favors the circulation of water through the aquarium. In addition, it also stirs the surface of the water to promote gas exchange and thus increase the oxygenation of the water.

It has a 3-stage filtration that is highly effective in purifying the water, and maintaining maximum water quality.

Eheim FLUVAL Series U internal filters have been on the market for a few years now. And despite retaining the same look, various tweaks have been made to improve the design over the years.

Hagen ‘s UK marketing manager, Paul Trott, says: “this is evolution, not revolution”. Evolved designs give the Fluval U range a bit of an edge in the competitive development race between filters.

Many aquarium enthusiasts know how to value a company that maintains a good design.

Many aquarist friends complain about this common problem. Finding a good product with just a couple of minor problems, which instead of being fixed, the company shuts down production and launches another new product, with new problems of its own.

We are going to see the characteristics of the fluval U filter and its “evolutionary” changes.

New Fluval Series U: Evolutionary Design

The first change in its design that was made was very interesting. A label on the front face of the filter, showing you how to remove the impeller and clean it. It also has a QR code that you can scan to watch it on video.

The second change has been made so that a new bag of filter media can be inserted. specifically designed to retain nitrate and phosphate (although it may not be the most suitable option for aquascapers).

The third tweak is in the media bin, and this might be a favorite of many an aquarist. A redesign has been made on the chambers that house the filter sponges. A white panel has been added to the bottom half to catch any loose debris as the filter is lifted from the aquarium.

The last modification involves access to the engine. It is now done in the same blue color as the inner chambers of the filter, to indicate that it can be removed. On the old model the motor was limited by the power cable that ran through the back of the filter. The new design allows the cable to be removed from its moorings, freeing everything.

Before going fully into the features, I remind you that you can take into account other types of filters. For example, there is the waterfall filter for a fish tank, also called a backpack filter.

Hagen Fluval U Series Technical Data

We start by looking at the technical characteristics of each of the models, and then we see a comparative table of the 4 models.

Fluval U1

river filter U2

river filter U3

Fluval U4


To start, we go to YouTube to get into the Fluval channel. We see the official video presenting the U Series of internal filters for aquariums.

We now see the main characteristics of this range of internal filtering devices.

We have already seen in the introduction that this filter is suitable for a marine fish tank, as well as for a freshwater one. Also that Fluval gives a 3 year guarantee. Let’s see other of its most notable features.

Adjustable flow, multi-outlet, addressable and with venturi

Also, on the Fluval U2, Fluval U3 and Fluval U4 internal filter models, the flow output can be set in three different ways:

  • Upper water outlet: This outlet achieves a very efficient circulation and oxygenation of the aquarium water.
  • Spray water outlet: In the intermediate area of ​​the filter there is a spray tube that maintains a smooth and constant flow of water. It is ideal in aquariums that have plants.
  • Bottom outlet: This filter has been designed with a third water outlet at the bottom of the filter. This output is designed to generate more movement in the deep zone of your tank.

A venturi is included in the Hagen Fluval U2, U3 and U4 models to provide proper oxygenation of the aquarium water. This has a dial with which you can regulate the amount of air generated.

fluval internal filter: silent filter

One feature that is important to many hobbyists is noise.

But more important is the lack of noise. The silence. In fact, it is characteristic of Fluval filters that they are absolutely silent. It is often one of the biggest concerns experienced and new hobbyists have. But you can be sure that this filter is very quiet.

hinged lid

The Fluval U internal filter has a hinged lid designed to facilitate quick and easy maintenance. You just have to open the lid of this device and remove the cartridges with the filter materials.

Multi-position placement

The design of this submersible filter allows the device to be placed both vertically and horizontally.

This quality means that it can be used as a main filter, but also as a support or complementary filter. It can be placed in the bottom of the aquarium in those tanks that need a reinforcement in the lower levels.

Its horizontal position is ideal in tanks with a low water level, even to create a decorative waterfall.

3-stage filtration system

Hagen ‘s fluval U filter has been designed with three phases that make up the filtering system of this device. The exception is the Fluval U1 model, which only includes the first stage.

  • The first phase of filtering performs a mechanical purification, retaining the largest solid particles with a foam pad.
  • The second phase of filtration accumulates the smallest remains, eliminating odors and favoring clearer water. For this , polycarbonate filter cartridges are used.
  • The third phase is made up of a biological filtration carried out by the colony of nitrifying bacteria. To favor this colonization there is the Biomax biological medium cartridge.

If you have any questions regarding friendly bacteria, and their nitrification process, stop by our blog to read the special article on aquarium cycling, where everything you need to know about tank maturation is explained.

Media cases are designed to retain loose debris, rather than returning it back to the tank.

Anti spill design

Another evolutionary change is in the media container, and it is a favorite of many aquarists. Many internal filters have a nasty habit of returning some of their contents back into the tank when you remove them.

Surely many know what I’m talking about, especially those who know almost everything about aquariums. You are trying to pull the filter out as smoothly as possible. As if it were a bomb. Just to tilt it a bit, and watch it pour the delicious brown slime back into the water.

To alleviate this problem, Fluval U has redesigned the chambers that house the filter sponges. Now, instead of little bits of debris collecting around the «gills» of each chamber, there’s a blank panel on the bottom half.

This retains any loose debris as the filter is lifted from the aquarium. It keeps it there until the filter can be placed in a bucket.

Easy impeller access

This may seem like a fairly minor advance, but believe me, it is not. Impeller fighting dirt and lack of maintenance is the number one cause of pump problems.

That also means it can be the cause of a lot of tanks going from overripe and leaky to suddenly over-leveling. Some thought that the origin could be in the lights for aquariums. But it often happens that the source of the problem is the impeller being dirty.

Anything that can highlight the importance of impeller cleaning and maintenance is fine. Especially for beginners in the aquarium hobby.

Maintenance and cleaning

To find out the best way to clean the fluval U filter, we turn again to the official brand, and its YouTube channel.

Opinions, reviews, ratings, reviews and reviews

On there are more than 34 opinions about the Fluval internal filter left by users. At the time of writing this review, the average score is 4.3 out of 5, a remarkable high, bordering on outstanding.

75 % of aquarium enthusiasts who have purchased this filter and who have left their opinion, have rated this submersible filter system with 4 and 5 stars. We highlight from your comments the virtues such as efficient water purification, easy cleaning, directable water outlets, its price or the versatility provided by its design.

Reviewing the Fluval reviews, some consider that the size of this filter is too large for an internal filter. Some users find that the «platform» system that fixes the filter to the aquarium is a bit «uncomfortable».

Among the worst reviews that can be found, there are some users who complain that their filter stopped working after a few weeks. Factory defects exist, and for those cases Fluval offers a 3-year warranty.

Other alternative

Conclusions about the Fluval U Series filter

We are facing a good, versatile and high quality submersible filter. The Hagen house supports the Fluval Series U brand, with a design that has been able to evolve since the beginning.

The general opinion leaves no doubt that this is one of the best internal filters. In fact, this model is considered by many aquarium hobbyists to be the best internal aquarium filter.

It is not surprising. The characteristics of this device make it a very versatile filter, which can be very useful in many situations.

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