Fluval C4 Power Filter Unboxed, Tested, & Compared (2024 Review)

Fluval have been a mainstay in the aquarium industry ever since they introduced the first 3-stage filter with synchronous motor technology back in 1975.

Known for making quality filters, and constantly innovating trying to bring us the best in the business, how exactly does the Fluval C4 power filter stand up to their legacy?

For this review, I bought the C4 and put it through its paces and compared it to similar competing HOB filters to help you know if this is the right filter for you.

Fluval C4 Review Overview

Fluval C4 filter overall review score chart

The Fluval C4 is hands down one of the best hang-on-the-back filters in the aquarium trade.

It might look like any other filter from the outside, but a quick peek under the hood will show you that its design is anything but ordinary.

This filter has a unique design that places each stage of filtration in its own little compartment, all of which can be quickly and easily pulled out for maintenance.

My only real gripe is that this filter has an external motor that needs to be primed before it can start suctioning water.

Bottom line: the Fluval C4 is really awesome and I highly recommend it.

Fluval C Series Power Filter, Clip-On Aquarium Filter
Fluval C Series Power Filter, Clip-On Aquarium Filter

Last update on 2024-07-17 / Commissions Earned / Images from Amazon Product Advertising API

This review is based on the Fluval C4. Fluval also makes C2 and C3 filters that are designed for smaller tanks.

How I Tested The Fluval C4

We purchased and tested five of the most popular hang-on-the-back filters on the market so that we could test them out for a head-to-head challenge. You can see that full comparison here.

We bought the following filters, all of which are rated for 70-75 gallon tanks (265-284 liter):

We unboxed everything, set them up, tested their GPH, examined and rated their features and let them run on my various tanks. 

GPH Testing

To figure out the GPH (gallons per hour) that each filter is able to pump out, we first filled a large, sturdy plastic tote with water. 

We attached each filter, one at a time and got it primed and running.

Then, we used a 4 gallon (15 liter) tub to capture the water cascading from the outflow. We timed how long it took for each filter to fill up the tub.

Each filter was tested four times, twice without any filter media and twice with all of the included filter media installed.

Then, we got to do math, so fun, let me tell you.

We converted each time into seconds, this made things simpler.

The two times for each category were added to each other and then divided by 2 to get the average.

The number of seconds were divided by 4, which gave us the gallons per second. Then, we took the number of gallons by second and divided it into 3,600, the number of seconds in an hour. That gave us the gallons per hour.

Here’s an example if the two recorded times were 01:30 and 1:20.

That would be 90 seconds and 80 seconds, respectively.

(90+80) ÷ 2 = 85 seconds to pump out 4 gallons of water

85 ÷ 4 = 21.25 gallons per second

3600 ÷ 21.25 = 169.41 gallons per hour

Here is our overall comparison of the five filters when it comes to GPH:

Hang on back filters gph with and without media comparison bar chart

The Fluval C4 came in last when it comes to just straight GPH. It’s still pretty close to most of its competitors, but the Marineland Penguin won the GPH contest by a landslide. 

But, this is only the beginning of our testing. There’s a lot more to examine than just pump speed when you’re trying to decide on a hang-on-the-back filter.

Feature Testing

Next, we took a look at the available features for each filter and ranked them based on the following criteria:

  • Biomedia capacity – does the filter have ample space for adding biomedia? The more biomedia a filter has, the more fish waste it can safely process.
  • Priming – does the filter require priming (filling the reservoir with water)? And, if so, is it difficult to get the pump going when you first set it up or do a water change? Or will the filter start on its own once the power is turned on? 
  • Filter media – what is the quality of the filter media that is included with the filter? Can you get up and running right out of the box or do you have to buy additional media?
  • Maintenance – how difficult is it to do monthly maintenance on the filter?

Fluval C4 Power Filter Reviewed

Fluval C4 on Katherine’s fish tank

What’s in the Box?

Fluval C4 unboxed
Fluval C4 Power filter box contents

I’ve got to say, unboxing this filter was pretty darn cool. Of all the filters on the list, this was the only one that I haven’t previously owned.

So, getting my hands on this bad boy was exciting.

Fluval has added some really innovative designs that no other filter on the market comes close to. 

Each kind of filter media has its own compartment and each compartment snaps in place inside the reservoir. Each compartment has a handle that sticks up so you can pull it out for easy maintenance.

Let’s look at the specific features we wanted to test out:

Biomedia Capacity

I was really impressed by the amount of biomedia that you can fit in this filter. It comes with a bag of ceramic media, plenty to get your beneficial bacteria going for a moderately stocked tank. This biomedia sits in its own chamber that’s topped with a cloth screen that is also a great place for beneficial bacteria to live.

As water moves through the filter, it drips water over the screen and down onto the biomedia. Plus, if you want to skip the activated carbon that comes with the filter, you can double the amount of biomedia by filling the carbon compartment with more ceramic media. I was adding seasoned media to this filter, so I added one small bag of media to the biomedia chamber and put another in the carbon compartment, worked like a charm.


This filter does require priming, so you’ll need to make sure that you fill the reservoir with water when you’re trying to get it started. I will say this, it’s very easy to prime this filter and get it up and running. Because of how the compartments snap together, there’s a small space to the left of the biomedia chamber that you pour water into.

So far, I have been very impressed with how little water it takes to get the pump going. I’ve got this filter set up on my 40 gallon (151 liter) angelfish tank and I’ve not had any troubles with it starting back up after short power outages. Although, I would definitely urge you to double check it when the power comes back on to make sure it’s started pumping again. But, overall, very happy with the priming on this.

Filter Media

Excellent media is included with this filter. You get a large bag of ceramic media and the drip screen for biomedia. There is also a huge reusable sponge for mechanical filtration and a large bag of activated carbon. I’m not a big fan of carbon, and usually don’t use it, but it’s there for any of the hardcore carbon lovers out there. All in all, very impressed with the media. 


Maintenance on this filter is a breeze. I especially like that you can pull the reusable sponge out so easily, since getting the gunk out of your mechanical media is the most common thing you’ll need to do. You can pull everything out individually and I love that the compartment for the biomedia slides out so you can set it aside while you’re cleaning everything else up. I would recommend having a bucket handy so you can put media in it to prevent drips. 

It’s also incredibly easy to make this filter shrimp safe. All it takes is sliding on a sponge prefilter over the intake and you’re ready to go with the adorable shrimps!

Overall Pros and Cons


  • Lots of space for biomedia
  • Innovative design that puts all the different media in its own compartment
  • High quality filter media
  • Easy maintenance
  • Easy to prime


  • Does require some priming

Fluval C Series Power Filter, Clip-On Aquarium Filter
Fluval C Series Power Filter, Clip-On Aquarium Filter

Last update on 2024-07-17 / Commissions Earned / Images from Amazon Product Advertising API

How Does The Fluval C4 Compare To Competitors?

Over score comparison bar chart for hang-on-back filters

I think the Fluval C4 performed really well in the head-to-head challenge. When I assigned ratings for each of the filters, the C4 came in second overall.

Frankly, if it had an internal motor that didn’t need priming, this filter would have easily been the winner.

I like the setup of the different filter media better on this filter than the other two top filters, the Seachem Tidal 75 and the AquaClear 70. Each kind of media has its own distinct compartment instead of being jammed all together.

This makes monthly maintenance on the C4 a bit easier.                                    

The Seachem Tidal does have a bit more room for biomedia, but the mechanical media on the C4 is much better. The reusable sponge on the Seachem is good, but the huge sponge that comes with the C4 just can’t be beat.  

I like the setup of the different filter media better on this filter than its competitors.     

I would also argue that the bottom two filters, the Marineland Penguin 375 and the Tetra Whisper EX, don’t even come close to competing with the C4. Both of these filters are self-priming, but they have much less space for biomedia and rely on filter cartridges.

I really hate cartridges because when you switch them out each month, you’re throwing away all the beneficial bacteria that grows on the outside of the cartridge. 

All a cartridge is is a bit of mechanical filter media on a frame that has carbon inside. You don’t actually need the carbon unless you’re trying to remove medications from your water.

There’s no need for fresh carbon every month and throwing away beneficial bacteria from your filter can harm your filter bed and cause ammonia spikes.

So, for these reasons, the filters that use cartridges stay firmly in the bottom and the Fluval C4 rises to the top.

Final Verdict

I don’t think it’s too hard to deduce from this article that I have become a big fan of the Fluval C4.

I think it’s a great fit for fish only and planted tanks alike. It’s also a good choice for shrimp tanks as long as you add on a sponge prefilter. 

I’ve been running this filter on my angelfish tank since the beginning of these tests and I have been very impressed.

There’s a ton of room for biomedia, especially if you utilize the carbon compartment to add even more media. 

You do have to prime this filter, but I’ve found that it’s pretty darn easy to get it going.

And maintenance is made so much simpler since it’s so easy to pull the different kinds of media out.

In my opinion, the Fluval C4 is a great choice for tanks 40-70 gallons (151-265 liters). 

All and all, this is an excellent hang-on-the-back filter and I highly recommend it.

I hope you find this article helpful.

I wish you and your fish the very best!

Fluval C Series Power Filter, Clip-On Aquarium Filter
Fluval C Series Power Filter, Clip-On Aquarium Filter

Last update on 2024-07-17 / Commissions Earned / Images from Amazon Product Advertising API

Katherine Morgan
Katherine Morgan

Hey, there! I'm Katherine from Northwest Florida. An aquarium specialist, I've kept tanks for over two decades, enjoy experimenting with low-tech planted setups and an avid South American cichlid enthusiast.

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