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Fluval U (U1 U2 U3 U4) Series Underwater Filter Reviews

Who does not like aquariums?

Alongside being used as decorative objects, aquarium tanks also provide a sense of serenity and calmness wherever they are placed.

But owning an aquarium tank can be high maintenance if they do not come with the right equipments and resources.

It becomes an even more difficult experience if you are someone who does not have prior experience to fish keeping before.

There are so many accessories that are important for aquariums, but the most important one has to be the water filter of it. In a way, the filter is the lifeline of your aquatic plants and creatures.

With that being said, we are going to review the Fluval U Underwater Filter. This is an amazing filter for all the right reasons and you are soon going to know exactly why.

We have also included an FAQ and info section at the end of this fluval u review so make sure to read that in order to learn more about filters in general.

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Things to look out for when buying a filter for an Aquarium

Let us look at the few key features you should look out for in a filter for an aquarium before you place that final order:


First and foremost, the size obviously matters a lot. Too much filtration is not necessarily a bad thing but if it comes at the stake of your plants and fishes being sucked into a vortex then no, that is not how it should work. But then again, under-filtering an aquarium sucks too (pun intended).

Just know that a bigger filter will definitely be more powerful and have a speedier suction than smaller ones.

So if you have small pets, you might want a smaller filtration system.


A good filter will be of great quality while also providing the best quality care for your pets and the tank itself.

A good filter should be able to easily get rid of all the ammonia and nitrite in a tank while also providing clear water by trapping in all the debris from before.

Fluval U (U1 U2 U3 U4) Series Underwater Filter Reviews

Video: “Setting Up: Fluval U series U2, U3, U4 Filter”

Not only is the Fluval U Underwater Filter very budget friendly, but it is also equipped with really amazing and unique features that is sure to catch your eye.

The filtration process of this Fluval Underwater Filter comes in 3 very efficient steps:

On the first stage, the filter produces a kind of foam from the foam pads that work to remove large unwanted particles from the water body. This helps the cleaning process to be much easier, less time and electricity consuming.

For the second stage, the poly/carbon cartridge of this filter model effectively traps all sorts of debris of any size. Because of this, the water appears more clear while also safer and clean for your aquatic creatures and plants.

Finally, on the third stage the unique BioMax system works to provide the best kind of biological filtration possible to keep the water flow and oxygen circulation effective for your pets and plants optimum care and safety.

The Specifications:

The installation and maintenance of this high quality no noise filter is quite easy and not a cumbersome experience at all. You can install this filter to aquariums where external filtrations are not possible.

You will be glad to know that this light weight filter of only 2.47 pounds comes with a 3 way control system with a variable output, as well as 2 foam pads and 2 poly or carbon cartridges for trapping debris efficiently.

The BioMax comes in 90 grams approximately, so you can easily use it for tanks of 34 to 64 gallons without having to worry about insufficient product quantity. This filter has dimensions of 3.5” x 12.5”.

The greatest feature of this Fluval Underwater Filter is that it comes with a life warranty of 3 whole years, within which you can easily change or reach out for customer feedback.

The U4 differs from its previous model, the U3 not only by size and capacity, but also in maintenance.

The U3 is smaller in size than the U4, where we can also state that the predecessors of the U3 namely U2 and U1 were even smaller. If your aquarium is more than 40 gallons, then you should go for the U4.

The U3 however had different cut outs for water flow, while the U4 has a plastic covering over half of its cutouts. But overall, the U4 is definitely better than its predecessors.

Frequently Asked Questions

1. Can I change the direction of the water flow?

There is in fact a small piece on every nozzle end which is meant for changing direction of the water flow to a minimum.

You probably will not be able to make drastic changes to the flow and direction of the water but you can somewhat tweak it.

2. What are the chemicals this filter can filtrate out?

The Fluval U Underwater Filter can clean out ammonia and nitrite without any difficulty at all. However, if your tank water also starts developing nitrate, you will manually need to change the water and refill the tank.

3. Is the Fluval U4 noisy?

No, this filter is actually pretty silent so you can easily install it in the aquarium in your bedroom. In fact, most submersible filters nowadays are quiet, and this Fluval Underwater is no exception to that.

4. Will it suck in smaller aquatic creatures like goldfish and snails into the filter?

This kind of depends on the size of the creatures actually. Very small aquatic creatures and/or loose plants might get sucked into the vortex created. But that is not the case for average sized ones at all.

5. Can this filter work in aquariums with sand in them?

Yes definitely, the filter will work absolutely fine. Since this Fluval U Underwater Filter comes with 3 filter settings, you can set it up to high, middle or low depending on what you prefer and if you have sand or not.

Pros and Cons


  • Very light weight product
  • Easy to assemble and/or install to aquariums
  • Efficient 3 step cleaning process makes sure to get rid of all impurities
  • Silent operation
  • Ideal amount of BioMax for water treatment
  • Comes with a 3 year life warranty
  • Budget friendly.


  • For some users, the top knob for flow changing feature was a little too tight to turn.

Video: “Fluval U-Series Internal Aquarium Filters”

Size & Filtration Capacity

This black Fluval Underwater Filter is quite a compact and sleek looking one, one that can easily blend in with its surroundings without appearing to be the odd one out.

If your choice of aquarium did not come with a filter, this Fluval Underwater Filter will easily compliment it.

This product comes in 4 different capacity styles;

Fluval U1

up to 15 gallons

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Fluval U2

12 to 30 gallons

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Fluval U3

24 to 40 gallons

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Fluval U4

34 to 65 gallons

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So you can choose one that fits your individual needs.


So there you have it!

We love this filter for all the right reasons and placing an order for it will definitely be the right option for you.

We hope you had fun reading this fluval u review and that it proved to be of help to you, your aquarium and its tenants. Happy fish keeping!

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Aaron Boyd
Aaron Boyd

Hello, I’m Aaron Boyd, the proud owner and author behind Aqua Movement. I hope my article was able to answer your questions. If you want to learn more about me, click the home icon above.

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