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Fluval

Fluval U3 Underwater Filter

  • Based on 2,156 reviews
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Availability: In Stock.
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Arrives Oct 19 – Oct 20
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Size: U3


Features

  • 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
  • Designed for freshwater, saltwater and reptile environments up to 40 gallons

Description

Fluval U3 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.

Brand: Fluval


Material: Electronic Components, Plastic


Tank Volume: 40 gallons


Product Dimensions: 12"L x 6"W x 5"H


Item Weight: 16 ounces


Specific Uses For Product: Indoor


Style: Underwater Filter


Color: Black


Included Components: Filter


UPC:


Brand Name: Fluval


Included Components: Filter


Model Number: A475A1


UPC: 015561104753


Number of Items: 1


Number of Packs: 1


Global Trade Identification Number: 53


Manufacturer: Rolf C. Hagen (USA) Corp.


Item Highlight: Designed for Freshwater and Saltwater Aquariums, Also Ideal for Terrariums and Turtle Tanks


Unit Count: 1 Count


Item Type Name: HAGEN FLUVAL U3 UNDERWATER FILTER


Manufacturer Part Number: A475A1


Warranty Description: 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.


Specific Uses For Product: Indoor


Vivarium Type: Aquarium


Pet Type: Fish, Reptile


Tank Volume: 40 gallons


Item Dimensions L x W x H: 12"L x 6"W x 5"H


Item Weight: 16 ounces


Product Style: Underwater Filter


Color: Black


Material Type: Electronic Components, Plastic


Frequently asked questions

If you place your order now, the estimated arrival date for this product is: Oct 19 – Oct 20

Yes, absolutely! You may return this product for a full refund within 30 days of receiving it.

To initiate a return, please visit our Returns Center.

View our full returns policy here.

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Top Amazon Reviews

    • Dependable, filters well and is absolutely quiet!
Size: U3
Purchased this filter nearly 3 years ago and it’s still going strong - we’ve not had one issue with it. Replacing the filter pads is super easy & they’re readily available. I love that it’s completely submerged so it is 100% quiet - no humming, bubbling or taking up space hanging off the back of your tank. Yes, it takes up some space inside the tank, but if you put it in the back corner you barely even notice it. You can direct how you want the water to flow out of the filter as well - from the top, middle or bottom - depending on the needs of your tank. I would definitely purchase again if for some reason this one craps out. ... show more
Reviewed in the United States on July 26, 2026 by NoNameRequired
    • Awesome underwater filter. No sound, dead quiet. Easy to clean and everything underwater. A+++
Size: U3
The Fluval U3 Filter is a very good underwater filter. It does a good job keeping the water clean. There are adjustable flow vents so you can adjust how you want the water to flow, and you can change it anytime with a turn or a twist. It is very quiet underwater, and changing the filter and filter media is quick and easy. You can do it all underwater pretty much. The filter media can add up if you don't buy it at a good price. Amazon does have some good pricing and some items are sold in bulk which make it cheaper. The bio-media you only change half at a time so to not destroy the cycle you have in the tank. The filter sponge can be squeeze rinsed in a bucket of tank water to remove the yucky stuff at put back. The poly/carbon cartridge i the only think you need to replace completely. You can try to squeeze and rinse out the poly/carbon in a bucket of tank water to buy more time with it, but the poly and the carbon side may both be less effective over time. As mentioned, these things can be purchased in a bulk quantity on Amazon for a great price saving you a lot of money. I recommend the Fluval U3 Underwater Filter because it works great and is very quiet where you here nothing. No more crashing waterfall from your overhang filter. Amazon usually has one of the best prices on the Fluval filters. If you have Amazon Prime, the 2-day Free shipping is a huge money saving plus. ... show more
Reviewed in the United States on March 5, 2016 by BBQ Corn Chips
    • New design of the filter now makes it one of the worst filters for a turtle that I have purchased.
Size: U3
Old review: Best filter for a turtle that I have found. This is my second one and would recommend the product to all turtle owners. . Revised review: This is now my third filter which was purchased to replace the second filter that I discussed above. I purchased the second filter on 10-24-16 and the third filter on 11-12-16. . The second and third filters are functioning below the standard of the original filter that I purchased a few years ago. The filter cartridge design appears to have been changed and the modified design restricts water flow considerably. As a result of the restricted flow due to a redesigned and smaller filter cartridge water inlet opening area, the filter clogs in half the time of the original filter and only half of the filter media sees direct inlet water flow. (When replacing the media only half of the media is dirty) I use this filter on a 50 gallon aquarium that is 60 percent full with water. This water level is sufficient for my turtle to swim and exercise. The original filter cleaned the water beautifully and required change out every 4-5 weeks, the new models require change out every 2-3 weeks. . I have modified the filter media housing (blue) by cutting additional openings to allow more water flow through the media. The modification has returned the functionality of the filter back to the original design and is working well. Change out every 4-5 weeks. . Do not purchase this filter for use with a turtle. As you can see I previously backed the original filter (design) for turtle use however, when this one fails I will look for another manufacturer and design. Very disappointing, I probably recommended and sold 5 of these filters throughout the years. ... show more
Reviewed in the United States on September 30, 2014 by Lee B.
    • Used in small outdoor pond
Size: U3
This is probably an atypical use for this filter, but I’ve placed it in a small (50 gallon) planted outdoor fish pond, and so far it's worked great. My requirements included: Submersible design that could sit at the bottom of the pond, low enough wattage to run from a battery pack (the kind you use when traveling with a cell phone or laptop), and adjustable flow so that I'm not blasting my little minnows around too much and kicking up too much sediment. The U3 met all of these and at just 6 watts, runs for about 6 hours on a 20000mAh battery. I leave it out in the pond and just run it when I begin to see debris particles floating in the water, which ends up being about 1-2 times per week. I don’t put any chemicals in the pond and have avoided algae blooms. Phosphates, ammonia, nitrates, and pH have remained good, according to weekly tests. The only surprise, compared to another brand of filter I tried, is that the U3 is buoyant enough that I have to put rocks on it to get it to sink to the bottom of the pond since the ground obviously isn’t glass like an aquarium, and it doesn’t stay suctioned to the bottom. Although the minnows can survive in very low oxygen environments, because the weather is hot and the pond is so small, I do run a separate USB air stone for them that lasts up to 48 hours on a battery pack. The combo of this and the U3 have kept the pond naturally pretty and habitable for small fish. The only downside is that the Fluval does suck up pieces of mosquito dunks as they dissolve... not sure if they still work if they're stuck inside the filter somewhere. I can’t speak to how it works in a normal aquarium setting, but it's a convenient way to keep a small pond looking nice and doesn't necessarily require an electrical outlet if you don’t want to run it constantly (I suppose I could run it as much as 6hrs every day if I brought the battery in to charge as soon as it was depleted). 6 month update: This little filter is a beast. I’ve bee using it clear the gunk that’d built up in the pond all winter, and it handles sediment, decaying worms, and algae without complaint. I’ve temporarily put activated charcoal in the center compartment instead of the ceramic pellets to try to deal with the water discoloration. ... show more
Reviewed in the United States on August 26, 2020 by mir
    • Cool but not great
Size: U3
I like the idea of this filter but it just didn’t do what I wanted. The amount of bio media that you can fit in these is so small you might as well just put filter floss in there. I also really wish you could use more than just one of the flow options at a time. The filter is really easy to clean and really couldn’t be any quieter but I think you’d have a hard time using this as your main. ... show more
Reviewed in the United States on October 6, 2022 by dillan
    • Nice filter
Size: U3
Works great for a 20 gallon tank with Molly's
Reviewed in the United States on August 18, 2026 by Lupe
    • Great filter, but...
Size: U3
I bought this filter because I currently own six Fluval FX6 cannister filters and have always been highly satisfied with this brand. I also needed an extremely quiet filter as this particular aquarium sits as the backdrop of my work desk. First the good: 1. This filter is utterly silent. It sits completely submerged in the tank and therefore cannot be heard at all. Every so often I have to check it to see if it's even still running. 2. This filter does an amazing job keeping the tank clean and clear. I set up an aquarium on the first day with Aragonite sand that had not even been rinsed, got the filter up and running and within 3 days the water was crystal clear (mind you, I am slightly over filtrating a 20 gallon with a U3). 3. It's very versatile in the different ways it can be configured. Now the (very) bad: The Fluval U3 Underwater Filter Poly/Carbon replacements are $8.99 EACH and you have to replace TWO of them once a month. I am kicking myself for not checking this prior to purchasing this filter, as that is just an outrageous amount of money to spend on monthly maintenance for one filter. What's worse is that I can't seem to find any off-brand options for the U3. This is a total deal breaker for me when I can get a Tetra Whisper In-Tank 40i for $16 that is equally as quiet and does a comparable job cleaning the water while only spending $8.45 for a year's supply of poly carbon replacements. For that simple reason, I'd take a hard pass on this one and go with something that has a more reasonable cost of operation, and that won't not set you back $20 per month to keep it going! ... show more
Reviewed in the United States on January 25, 2023 by J Reine
    • Love Fluval
Size: U3
I live Fluval as a brand. They are the one I trust the most. This is my second underwater filter, only in that I had to go with a bigger tank. It’s easy to set up and the underwater part makes it silent!
Reviewed in the United States on July 12, 2026 by faquinones

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