Fluval FX4 Review – the right canister filter for you?

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When setting up an aquarium, choosing the right filter can decide whether your tank goes on to mature and thrive or you have continuing problems down the road. Filters are the lungs of your ecosystem so you should always make sure you fully understand how to choose and care for them!

Fluval FX4 Review

Since proper filtration is essential to keeping your fish healthy, a high-end canister filter is the best way to provide the three stages fish need for survival. Fluval is a trusted brand that has been around for decades and in that span, they have honed their aquarium technology to a fine art.

The Fluval FX4 is my #1 choice for mid-range aquariums on the larger side (100-250 gallons). While the FX6 is perfectly designed to handle the largest tanks the FX4 provides exactly the same benefits in a scaled-down package!


  • Flow control allows the FX4 to be used with aquariums as small as 55 gallons
  • Smart Pump both auto-purges air bubbles and moderates power usage and flow for maximum efficiency
  • Incredibly easy to both setup and maintain despite its size


  • Pricier than some other brands rated for the same tank size
  • Included Fluval Biomax doesn’t have the right pore size for anaerobic bacterial activity
  • Extremely noisy when purging air every 12 hours

Fluval FX4 Features, Benefits, And Specs

Near-Silent Operation

Filters can be noisy and disruptive to a quiet environment such as a bedroom or home office. Canister filters, which typically sit within the stand, can be even more problematic if the stand resonates with the hum, as wood often does.

The Fluval FX4 is not only innately quiet-running but also has rubberized feet to prevent the unit from causing the entire stand or hardwood floor to noisily vibrate. It’s not completely silent as the pump is powerful, but it’s on par with the fan in a running laptop or other electronic device.

You should know that every 12 hours the FX4 (and FX6) auto-purges the water lines for possible air buildup. This takes less than 2 minutes but is rather noisy and if you haven’t read your manual you might think you have a faulty unit!

Smart Pump Power Monitoring 

Canister filters not only run continuously but their larger pumps use more power than hang on the back filters do. For comparison, the Aqueon QuietFlow Power 55/75 Filter uses only 14.5 watts of power. However, it only turns over 400 GPH at maximum flow.

That’s why Fluval designed the Smart Pump monitoring system to ensure that flow is continually being monitored via a microchip computer built into the unit. Rather than working continually at full power, the pump adjusts itself for maximum power efficiency.

Generous Media Capacity

The Fluval FX4 includes room for approximately a gallon of biological media, competing very favorably with the 1.5 gallons offered by the FX6. Biological media is the most important because that’s where toxic nitrogenous wastes (ammonia, nitrite) are broken down into less toxic chemicals.

The included foam media has 1,400 cm2 of surface area for capturing suspended detritus and additional living space for beneficial microorganisms. And having media separated into two stackable baskets ensures flow moves through the entirety of the media compartment. 

Some canister filter designs use a single-stage storage method. This can result in water bypassing if your media gets clogged or the pump’s flow is weak.

Generous media capacity is very important for several reasons. For starters, more media gives you a greater cushion to work with in case of a sudden issue, such as a fish death that happens while you’re on vacation. Extra media capacity also allows you to not only use typical foam, activated carbon, and ceramic/lava rock but specialty media as well.

If your fish are especially sensitive to ammonia, such as stingrays or invertebrates, the FX4 has plenty of capacity for ammonia-absorbing resins. You can also target silicates, phosphate, and other problem elements with this canister filter!

Compact Design

Space is always at a premium when setting up an aquarium. Thankfully, the FX4 is a very modest 15.6 x 15.6 x 17.7 in, ensuring the majority of its internal volume is dedicated to media.

Self Priming Design & Maintenance Reminders

Less advanced canister filters can be a bit of a hassle to restart each time you need to do maintenance. You often need to start the unit filled with water and manually expel air from the lines. And even if you do things right there is a period where the pump noisily struggles to 

Thanks to the Smart Pump system the Fluval FX4 will auto-purge the lines of air so long as you start the unit topped off with water. And each time you own the canister a timer resets to remind you when it’s time to perform monthly maintenance! 

Comparing the Fluval FX4 to the Fluval FX6

Since the Fluval FX5 is no longer in production it’s best to compare the FX4 with the FX6. Previously there was a bit of a gap in Fluval’s lineup as the Fluval ’06 lineup doesn’t provide models for tanks between 100 and 250 gallons. 

The FX6 is the obvious choice for aquarists looking for the most media capacity and water flow. But how do these two models compare with one another? The FX6 also differs slightly in the amount of media provided. Both models include Fluval Biomax, Carbon Foam, and Standard Foam. However, the FX4 has room for only four of the standard Foam inserts while the FX6 includes six units.

Fluval FX4/FX6 Comparison Table

Fluval FX4Fluval FX6
Aquarium RatingUp to 250 GallonsUp to 400 Gallons
Biological Media Capacity1 Gallon1 Gallon
Power Usage30 Watts43 Watts
Gallons per Hour700 GPH925 GPH
Dimensions15.6 x 15.6 x 17.7"15.75 x 15.75 x 20.8"

Since the units are identical in terms of maintenance and design the FX6 is the clear choice if your aquarium is on the larger side. The Fluval FX4 is perfect for both mid-range (100-250 gallons) and smaller 55-100 gallon aquariums that need more powerful filtration!

If you are interested in the FX6 check out our complete Fluval FX6 Review!

Set Up & Cleaning/Maintenance

As efficient as the Fluval FX4 is at maintaining water quality it is also intelligently designed to be easy to both setup and maintain. The unit can be up and running within minutes of opening the box because the Smart Pump is self-priming.

A cleverly placed Purge Valve aids water changes by allowing you to drain water directly from the base of the filter. Also included with both the FX4 and FX6 and designed specifically to work with the Purge Valve is the Fluval FX Gravel Vac. This accessory attaches to the Purge Valve once you’ve disabled the filter intake.

Water is drawn through the vacuum into the filter and then sent right back into the aquarium! No more bucket brigade or having to make constant delicate adjustments to tap water to get the chemistry right! 

Should you need to access the interior the Aquastop Valves and screw clamps are simple enough to release. As mentioned earlier there are two media baskets, which have T-handle releases for lifting them out of the unit.

The FX4 also has a pre-filter in front of the water intake line to prevent heavy debris from gunking up the interior. This allows you to go longer without having to messily disrupt the interior!

Final Thoughts On the FX4

Aquarium filtration is one area where it really pays to be diligent in researching your needs. As the lungs of your ecosystem, the filter should have more than enough capacity to keep your fish healthy over the long term.

And since canister filters have the best track record in this regard, I personally recommend the Fluval FX4 as the best mid-range model available. It provides a generous, customizable media basket that’s simultaneously easy to access and maintain.

The unit is quiet and compact and intelligently designed to aid aquarists in weekly and monthly maintenance as well! Fluval Hagen’s really done well at perfecting their FX lineup and the FX4 will meet the needs of even the most demanding aquatic animals.

What Should I Consider Before Deciding on a Canister Filter?

Most aquarists start out with hang on the back style power filters. However these units have several limitations. The most important of which are that the media compartments are designed to hold proprietary bags and the small pumps don’t provide an especially high amount of water turnover.

That’s why I always recommend canister filters for optimal water equality. When I’m shopping for a canister filter I consider the following:

How Much Filtration Media Capacity Do I Need?

Filtration media is the most important aspect to running your filter because it determines what sort of pollutants can be strained out. Power filters are inexpensive and convenient. However canister filters provide ideal water quality and unlimited choices thanks to their expansive media baskets.

A proper canister filter has room for mechanical, chemical, and biological filtration. And the more animal biomass you have the more media you’ll need to break down nitrogenous waste and trap detritus.

How Much Water Turnover Does It Create?

When it comes to gallons per hour (GPH) more is not always better. Too much flow can make it hard for smaller fish to swim and for decorations to stay in place.

Most canister filters allow you to adjust the flow to your preferred level. However there’s little point paying for more flow than you need so knowing the GPH rating is crucial to making a smart purchase.

High water flow is especially important if you’re keeping messy organisms like large cichlids or turtles. A canister filter rated for a 200 gallon aquarium may only give you the equivalent of 100 gallons of cleaning power if you’re keeping several turtles together.