Fluval U2 internal filter review


The Fluval U2 internal power filter gets some tough testing from Jeremy Gay in our review.

Internal filters are the most popular type of filter on the market, with the biggest choice of models and probably the most varied group of people using them — from beginners to nano owners to quarantiners to big cichlid and oddball keepers. Internal filters are readily available and have to cope with a huge range of situations. What I want from any internal is versatility and reliability, as they are often the only form of filtration used in the aquarium.   

What makes a good internal power filter?

There are several characteristics I feel I want when buying an internal power filter. They are:

Adjustable flow

Depending on the type of fish that you keep, you are either going to want slow or fast flow and a flow control that is simple but effective.

Directional flow control

If your filter fits into the rear corner you may not want a water flow that is only unidirectional. Having the option of being able to direct the flow left to right and/or up and down gives greater flexibilty and is a function easily achieved by the manufacturer at little cost.

Venturi option

A Venturi uses the throughput of water from the filter outlet to suck in atmospheric air and produce bubbles. This free form of extra aeration is of  benefit to coldwater aquariums, heavily stocked aquariums and basically any aquarium containing fish, where corals or plants don’t dominate (air bubbles irritate corals and drive off CO2) You can normally adjust the amount of air being produced too. 

Double sponges

For alternate cleaning. If sponge is the only media available for your internal filter it is tasked with performing the roles of both mechanical and biological filtering. The sponge needs frequent cleaning, yet cleaning risks the bacteria colony. The solution is two sponges, so that they can be cleaned alternately, or even replaced alternately, and the bacteria remains intact.

Media options

The ability of some filters to take extra media is a feature that has definitely come to the fore in recent years. The more media the better, as it means more cleaning power and more waste reduction power.

A simple media upgrade is the polyester pad, though with some models it appears to be more about selling the pads as a disposable and hence replacable item more than it actually appears to benefit.

Carbon inserts have visible benefits as they remove dyes from water, odours and traces of medications. They remove chlorine too. But, for me, the best media option of all is extra biological media. As with the double sponge scenario, one doing the mechanical filtering and leaving the other to do the biological filtering, is better than one media that is tasked to do two jobs. In my book, extra biological means a better filter.


I contacted every manufacturer or supplier of internal filters I could think of and invited them to send in a filter for review. So that I could compare models like for like I requested their choice of model for an aquarium 60cm/24” long with standard dimensions and a volume of between 50-70 l/11-15 gal. Fourteen filters arrived at the office, including popular models from well known manufacturers and a few models that you won’t see in all the shops.

The models also varied from retro relaunches from the likes of Eheim with their Eheim Pick up, to the JBL Cristal Profi I, new in 2008, and the Fluval U, new in late 2008/early 2009.

How I tested

During the test I was still in the early stages of setting up my fish house. This put me in the convenient position of having nine 24” tanks available in which to test filters, with seven containing no fish — so I would do two batches of tests, seven at a time.

This meant that they would be tested in real situations, with real filter muck, and the results could then be compared.

To accelerate the test from a few months to a few days, I cleared out all my other external and internal filters and collected the dirt in a 10 l/2.2 gal bucket. I then added some stuff that typically gets caught up in and clogs filters.

In with the dirt went Java moss, slime algae, hair algae, Vallis leaves, general plant matter and baby Java ferns. I stirred the muck, split it 14 ways, put a filter in each tank, and added the muck. Once in the tank I stirred it all into suspension and observed how the filters did at clearing it. Once clear, I could then maintain the filters and see how the muck had been trapped in each one.

How the Fluval U2 fared

It's very good...

Being the most up-to-date model in the test, Fluval manufacturer Rolf C Hagen have had the longest time to get their new model right. Having the very popular Fluval and Fluval Plus internal filters to fall back on for experience helps greatly too, as both were seemingly bomb-proof.

In terms of features this filter has a three-way flow, offering water flow from the top outlet, the bottom outlet or the built- in, vertically-mounted spraybar in the middle. The flow from the top and bottom outlet is directionally controllable, and a Venturi is supplied.

Next, and very important, are the media supplied. This filter comes with two sponges for alternate cleaning, a polyester and carbon cartridge combined for extra mechanical and chemical filtering, and best of all, separate Biomax biological media. What’s more, you access all the media for replacement or cleaning through a flip-top lid and they slide in and out easily on vertically-mounted media trays.

The whole filter hinges forward for complete removal from the aquarium, leaving the fixing bracket in situ.

Output is 400 lph for 5w of electricity.


Not much really, other than the flip-top was a bit stiff on our new model and impeller access isn’t as accessible as it was on the previous Fluval plus model.


Best internal filter in the test.

Product: Fluval U2 underwater filter

Price: £36.21

Rating: *****

Output: 400 lph

Energy: 5W

Lph/watt: 80

Tank volume: 45-110 l/10-24 gal

No. of sponges: Two

Carbon: Supplied

Bio media: Supplied

Venturi: Yes

Directional flow control: Yes

Adjustable flow control: Yes

This article was first published in the January 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.