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Fluval U4 Underwater Filter, Freshwater & Saltwater Aquarium Filter, Black

Based on 3,270 reviews
Condition: New
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At a glance

Condition
New
Size
U4
Model
A480A1
Capacity
240 liters
Dimensions
5.12"L x 4.72"W x 14.17"H
Weight
1.12 kg
Material
Metal
In the box
Filter
Warranty
3 years; Fluval U Series Power Filters are guaranteed against defects in material or workmanship under normal aquarium usage and service for 3 years. We offer a 3 year guarantee on all nonreplaceable parts which means that the filter will be repaired or replaced at the manufacturer's discretion, free of charge, when the complete pump is returned with proof of purchase and postage paid. This warranty does not apply to any filter that has been subjected to misuse, negligence, or tampering. No liability is assumed with respect to loss or damage to livestock or personal property irrespective of the cause thereof.

Features 5

  • Use as a primary filter for smaller aquariums, or supplementary filter for larger aquariums
  • Sleek design with easy grip water control panel and redesigned media cartridge to trap more debris
  • Convenient flip-top lid allows for quick and easy access to filter cartridge for maintenance or replacement
  • Position horizontally for shallow tanks, larger tanks with low water levels, or to create a decorative waterfall feature or vertically against aquarium wall to create currents or customized flow patterns

Description

Fluval U4 Underwater Filter is ideal for use in all aquarium or reptile environments. Use the underwater filter as a primary filter in smaller aquariums or as a supplementary filter for larger aquariums. Fluval Underwater Filters with three-stage filtration are ideal for use in fresh, marine or reptile environments, and are also great for tanks with low water levels. Three-stage filtration with BioMax for superior aquarium water quality, flip- top access for quick and easy cartridge replacement, and three-way flow control make the new Fluval U Series the benchmark in underwater filtration. Stage 1 foam removes large particles. In stage 2, the poly/carbon cartridge traps fine debris, improves water clarity and removes impurities. In stage 3, BioMax provides optimal biological filtration efficiency. For more fish tank accessories, check out other Fluval products including aquarium filters, aquarium heaters, aquarium air pumps, aquarium lights, aquarium rocks, aquarium thermometers, and aquarium stands. Fluval has a wide variety of products that are perfect for your pet fish to enjoy in any large fish tank, small fish tank, saltwater tank or freshwater tank. Fluval broke onto the aquatic scene in 1975 by introducing the world’s first 3-stage filter with synchronous motor technology – a breakthrough that is still applied in the industry today. Since then, Fluval has built a legacy around this pioneering spirit, relentlessly adapting in a changing world to launch innovative aquarium products that remain at the forefront of technology, and which set the bar in quality, style and functionality. Fluval’s mission is not just about building products that mimic the natural world, but inspiring aquarists of all skill levels to enjoy and respect the hobby for generations to come.

Specs 23

Brand Fluval
Product Dimensions 5.12"L x 4.72"W x 14.17"H
Material Metal
Capacity 240 liters
Installation Type Underwater
Purification Method Biological
Power Source Corded Electric
Item Weight 1.12 kg

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If you place your order now, the estimated arrival date for this product is: Friday, Nov 13

Yes, absolutely! This item's return window has been extended for the holiday season: FREE refund/replacement until Jan 31, 2027.

To initiate a return, please visit our Returns Center.

View our full returns policy here.

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Top Amazon Reviews


Used in 50 Gallon Turtle Tank
Size: U4
__________________________________________________________________________ UPDATED 7/26/12-- see bottom part of entry-- upgrade from 3 to 5 stars __________________________________________________________________________ Original review June 2012: I'm writing a long review because I found it helpful to hear others' experience specifically with turtles. They are a little different than fish when it comes to what works. I have a painted turtle, about 5-6 inches long right now. We have an algae eater too who helps keep the tank clean (we call it a sucker fish, not sure what its real name is). We also periodically add small minnows and goldfish to entertain the turtle and for food if we forget to feed her or go on vacation. Since she's primarily a water turtle, her tank consists of mostly water with a few haul out spots. We just upgraded from a 20G tank to this one. Previously we've always used the Whisper Quiet filters with great success. People would rave about how clean the turtle tank was, and we could pretty much ignore it (don't worry about changing water, etc.) In the 20G tank we had the 20-40G Whisper Quiet in about 16G of water, no plants, a few submerged items, and a floating plastic "log" for basking. For the 50G, we added a few more submerged and surface items, but all are plastic or rocks, nothing living. Upgrading to the 50G tank (with about 40-42G water) posed a problem for me. All of the larger hang on back filters required the water level to be higher than I wanted, since the turtle needs some land, and I didn't want it climbing out. I tried rigging a few up-- the larger Tetra one worked very well, but I had to rig it inside of the tank, and it just took up too much room. I decided to try the underwater Fluval for two reasons-- never need to worry about water level, and because I'm not willing to invest in a canister filter. PROS: --silence. it's golden --ease of changing filter media (it's really simple, you can lift it up so you don't contaminate everything or spit junk back out) --will never dry out or burn out if you lose power and it restarts (since submerged) --ability to place where I like in the tank for optimal filtering and flow to stir things up --ability to change parts of filter at a time to preserve beneficial bacteria --foam pad separate from other pieces so you can rinse that and not everything --no splashing, so no hard water stains on side of tank CONS: --I want to love this filter and I hope it works long term. We used it in a tank that had been recently filled and set up, with the Tetra running for about a week. Initially this worked really well, but about a week into it, the water turned nasty green. First off, TURTLES ARE DIRTY. By nature I don't think this filter accomodates that. I'm holding off judgement since it sometimes takes a while to get things right when you set up a new tank, as it happened before with other filters too at times. However, in this case, I think it's because of a lack of active carbon in the filter. There is just a tiny bit there, and in my experience (6 years with this turtle), the active carbon is ESSENTIAL to keeping them from getting stinky and gross. I decided to remove the bio pellets from the middle part (step 3) since I never had them before in my tanks anyway, and instead fill that space with one of the tetra filters filled with activated carbon. It's been about 10 hours. The nastiness is gone and the tank looks much better. There still seem to be a lot of particles in the water, but I am continuing to work on that-- not sure if I can blame it on the filter at this time. I will keep adjusting, because I think this filter is worth figuring out how to make it work for turtles. I gave it three stars because it advertises that it can be used for my purpose, but I don't think it comes through with the media it uses. I also think their media replacements are expensive relative to other filters (which is especially frustrating when they aren't even working for me). However, if the makeshift adjustment of using other carbon filters inside the third chamber is effective, I would upgrade to 4 stars. If I get to perfectly crystal clear water without the need to pay attention to the tank every week (yes, it's possible with some cheap filters), I'd go to 5 stars. I ask a lot from my filters! ___________________________________________ UPDATE 7/26/12 This definitely deserved the upgrade to five stars. I had to leave for three weeks after adding the Tetra carbon filter to the center in place of the bio pellets, so I filled it with a bunch of minnows for her to eat over time, then came home to a tank that was still crystal clear after 24 days. (Trust me, I was almost afraid to look at the tank when I returned for fear of how gross it might be.) A few minnows were still swimming around happily too. I went ahead and swapped out the carbon filter, but it looks like I can simply wash out the outer sponge and pathetic carbon insert supplied with it periodically and replace the Tetra carbon insert in the center chamber monthly. I still don't like that I had to adjust it, but I don't think there's a great product out there for turtles, and this is working VERY well. Plus, I am now purchasing the cheaper Tetra filters, so my ongoing costs will be less. If you go this route, it does seem like a very tight fit to fold over the Tetra filter and slip it in the middle chamber, so don't be shy. Just wiggle it as you push it in and be gentle. All that remains to be seen is longevity, but for now I am one happy consumer! ... show more
Reviewed in the United States on June 27, 2012 by Patricia L. Horn

Awesome Upgrade for my Turtles
Size: U4
I have two small aquatic turtles - Texas Map Turtles; males max out at about 4 1/2 inches or so. I recently upgraded them to a larger fish aquarium rather than a shallow water turtle tank. I added the Fluval U4 and so far, after several weeks, I am so happy I "invested" in this filter. Turtles are dirty animals. They make a huge mess of their tank and turtle-parents like me are challenged to keep their water clean. A Tetra waterfall filter did an acceptable job in their shallow water tank so long as I changed the water twice a week. My first upgrade was to a fluval external filter, which was OK, but just ok. Within a few weeks I found that I was still needing to change the water more than once a week, and that the tubing, first from the tank to the filter, then also from the filter back to the spray bar which returned the filtered water to the tank, would fill with gunk which provides a lovely breeding ground for planaria and other aquarium unpleasantness. And once again, I need to emphasize that turtles are Gross. Srsly. Since adding the U4, my bad boys' water is keeping so much cleaner. This does not mean that I can just go ahead and neglect the tank! But I am truly happy that I bought them a better filter. With this filter I can have the tank filled almost to the top with water - I just need to have a low enough that the turtles can't escape the tank when they are on their basking platform (I also added an additional "guard rail" to keep them in, but that's another story entirely . . .). The water is staying cleaner, longer. I also appreciate how well the filter moves the water around the tank. The filter gives the choice of three circulation options. For the turtles, I chose the water to return from the bottom outlet for maximum agitation. Still water gets dirtier, faster, and so I am Loving this option. I noticed another review used the phrase "plug and play" for this filter. Can't say that better, and so I say it the same. I am going to actually get another of this model - but the U3, I think, for my son's fish tank. It currently has a hang on filter which I find annoying because when the water gets a bit low the filter starts sucking air - which of course says to me that it's time to give that tank some attention . . . but aside from the air-sucking thing, the hang on filter is Loud and there are no speed flow options on that particular filter. I think my son's fish will also appreciate the quiet, and they will appreciate the internal flow of the water, which I think for them might come from the top, for maximum oxygen. Five stars. Srsly. ... show more
Reviewed in the United States on July 12, 2012 by Marianne Hill

Quiet filter and Great for Turtles!
Size: U4
FINALLY a filter that works for turtles. This filter can mount sideways, which is great news for turtle owners who only fill the tank halfway to allow room for a turtle basking platform. Most filters are designed to draw water from the very top of a full tank. This filter can be mounted either vertically or horizontally to accommodate a lower water level. This is also the quietest filter I have ever owned. The entire filtration process occurs below the water level, so there is no running water sound, only the soft hum of the impeller drawing water into the unit. Also, all water says inside the tank, so there is no concern with leaks or broken hoses as you might expect from an external canister filter. I have this filter set up in a 40 gallon tank with one medium sized red-eared slider. It does a great job of keeping the water clear, and it is easy to maintain. With more turtles or a larger tank, I would recommend using more than one U4 filter to ensure enough filtration power. Overall, I am very happy with this filter and would definitely recommend it to fellow turtle owners. ... show more
Reviewed in the United States on October 14, 2014 by goat8485

The U4 filters the water well and has good flow, but it has two main flaws
Size: U4
The U4 filters the water well. Its silent and has good flow, but it has two main flaws. The first one is too much dirt drops out of the filter trying to get it out of the tank to clean it. I figured out a way around this by placing the U4 in a large filter sock before lifting it out of the tank.The second issue is that the sponge and carbon filter pad compartment should be enclosed so you can put whatever media you want in it just like the center bio compartment. The design of the outer compartments force you to use Fluval's weak carbon pad instead of being able to jam a bag of carbon in it or a purigen pouch if you wanted. I've gotten around this by using generic non-carbon fine filter pads along with the coarse sponge that comes with the U4 in the outer compartments and placing a chemi-pure blue nano pouch in the center compartment with the Fluval bio-max biological media. ... show more
Reviewed in the United States on April 28, 2016 by Frank

Great Filter
Size: U4
After a week of usage, I can safely say I am amazed by options offered by this filter. I have a 30 Gallon Tank and Turtles and the u4 is doing a great job of keeping the tank clean. This filter is placed inside the tank either straight up or laid down. It has three options for filtering. A, is the strongest current, B, utilizes the filters spray bar for an even flow, and C takes care of the bottom of the tank. I was pretty amazed at how much power this filter has, it creates a mini river flow in the tank. This helps debris circulate towards the filter and provides my turtles some fun as they exercise their way through the current. The filter it self has a 3 stage filtration. It uses 2 foam pads and bio nodes to help keep the tank clean, there are replacements available online if you so need it. On top of that accessing these foam pads and bio nodes is made easy with the filters flip top. Another option this filter has is outputs for a bubble wand or any other tube like apparatus. And one last cool thing that this filter comes with its own detachable "holding cradle with suction cups." You tilt it forward slightly and you can remove the filter from the tank for easy cleaning. Overall I give this filter 5 stars for its sleek design, many options, and great filtering. I do recommend this filter for both turtles and fish owners. ... show more
Reviewed in the United States on September 5, 2011 by Yen

Don't waste your money
Size: U4
Cheap piece of plastic. Fell apart and broke after a few months. The bottom suction clip broke the day I got it but needed it that day. Soon after piece after piece broke then one day (5 months after purchasing) it quit working. Pain in the butt filter, don't waste your money. Also I bought the asurion warranty with it and it's a run around game for a refund or replacement. Ridiculous. ... show more
Reviewed in the United States on August 15, 2026 by Amanda rose

Works great as a secondary filter
Size: U4
I wanted more filtration for my cichlids tank. I was skeptical at first because of its bulky size (photo attached to show how big it is in the tank) but figured if it worked well I could look past that. I had a corner where so much waste would accumulate and I would have to clean it out every other day but literally as soon as I would clean it there was waste there within minutes. It felt endless. This has totally cleared up that spot. I took it out after 24 hours to look at the sponges and they were filthy lol... anyways super easy to clean (easier then siphoning waste everyday😵💫) and works great! Totally recommend as a second filter (can't say how it works as the main filter) ... show more
Reviewed in the United States on February 11, 2025 by Florina

Conflicted
Size: U4
I bought the u2 for my turtles 20 gal tank, I knew it probably wasn’t going to be enough to handle all the mess but she is very small and I didn’t want her to drown from a strong current from bigger filters. At first it did pick up pretty much all crumbs, but it will make weird gushing sounds if the lid is closed so I have to leave it open. I’d also like to say the tank isn’t completely full so it’s not fully submerged but it stills works and it’s completely quiet which I like. Over time it doesn’t seem to be doing the job completely but it never did anyway. Eventually I bought the u4 but when I put it in it was LOUD and making gushing/bubbling noises. I’m not sure if I got a dud or if the amount of water I had in the tank wasn’t enough. I returned it but I was hoping it was going to be as quiet as the u2. It was super long too but that’s expected. The u2 is the only filter I can use right now for the size of my turtle and because it’s pretty much silent. Eventually I will upgrade to the fx4. It does suck cleaning this and it’s kinda hard but I guess that’s just part of being a turtle owner. I am considering the u3. ... show more
Reviewed in the United States on July 5, 2024 by Nat
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