Fluval U4 Poly/Clearmax Filter Cartridge

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Keep your Fluval U4 Underwater Filter working effectively and help to control green water and algae growth with Fluval U4 Poly/Clearmax Filter Cartridge. Fish waste, uneaten food and other organic matter can build up in your aquarium water, causing algae growth and increased phosphate, nitrite and nitrate levels. Raise these levels too much, and they can be harmful to fish. This filter cartridge helps to absorb and trap these pollutants, protecting your aquarium and those who call it home. Designed for the Fluval U4 Underwater Filter, Fluval U4 Poly/Clearmax Filter Cartridge supports crystal clear water, helps to control odor and works to reduce overall aquarium maintenance.

Key Benefits
  • Helps control green water and unsightly algae growth for a clear, beautiful tank.
  • Chemical filter media helps to absorb and trap phosphate, nitrite and nitrate.
  • Encourages crystal clear water, helps to reduce odors and encourages the ideal conditions for fish and plants.
  • Helps to reduce overall aquarium maintenance, so you can enjoy your tank more and worry about it less.
  • Convenient two-pack gives you one cartridge for now, and one for later.

Compatible Products

Ensure your system runs smoothly with Fluval Underwater Filter.

See all items by Fluval

  • Item Number
  • Aquarium Type
    Freshwater, Saltwater
  • Fish Type
    Tropical Freshwater, Marine Fish
  • Material

Replace 1 Poly/Carbon cartridge every 2 weeks.

Do not replace all filtration cartridges at the same time. Alternate filter cartridge changes so that some older cartridges remain in use. This ensures the re-seeding of beneficial bacteria, so that colonies discarded with older cartridges will be replaced by vigorous new ones. To enhance bacterial action, pour a dose of biological aquarium supplement onto your filter cartridges each time you change the media.

  • Unplug the power cord.

  • Open the filter cover (To make the operation easier and in case the filter is positioned under an aquarium frame, tilt the filter forward until it stops but make sure there is enough electrical cord to allow the filter to pull forward).

  • Slide the foam cartridge frames out of the filter body. Keeping them vertical to avoid releasing the debris collected inside the debris receptacle. Place them in an empty bucket.

  • Remove the poly/carbon cartridge from the cartridge frame.

  • Remove the foam. Rinse the debris receptacle and the foam pads using water taken from the aquarium or replace one of the foams following the recommended replacement schedule. Do not use tap water as this will kill the beneficial bacteria in your foam. Note: Never use soap or detergents as these products could be harmful to the inhabitants of your aquarium.

  • Replace the foam pad. Make sure the foam is positioned behind the tabs on the sides of the frame.

  • With the carbon side facing out, re-insert the poly/carbon cartridge by inserting the lower end of the cartridge into the lower end of the cartridge frame then snapping the top side of the cartridge under the two tabs on the top side of the frame. Do not press too hard, as the cartridge must sit on the top of the tabs on the sides of the frame. Hint: For best results replace 1 poly/carbon cartridge every 2 weeks.

  • Slide the BioMAX cartridge out and place it in the sink or bucket.

  • Pull open the tab on the side of the cartridge, opening the cover. Rinse the BioMAX with aquarium water, never tap water, or replace half the quantity every 5-6 months. Never replace all the BioMAX nodes at one time, as valuable colonies of beneficial bacteria would be lost.

  • After rinsing, put the BioMAX back in cartridge and close the cover. For best results completely fill the cartridge with BioMAX nodes but allow the cover to close easily.

  • Re-insert foam cartridge frames into the filter with the poly/carbon pad facing inwards. Re-insert the BIOMAX cartridge into the filter making sure to properly align the hole at the bottom of the cartridge directly on the top of the impeller opening.

  • Close the filter cover and if required snap the filter body back into position on the filter bracket.

Rating: 2.8 of 5 stars 6 Reviews 6 Reviews 2.8 out of 5 Stars

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Rating: 2.8 of 5 stars 2.8 / 5

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6 Reviews

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  • 1 out of 5 stars

    weird red brownish stuff on new items

    By Disappointed on Sep 20, 2020

    Why did my cartridge arrive with this red tint to it???? So confused

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  • 5 out of 5 stars

    Love these filters

    By Bridgett on Aug 24, 2018

    I have always paid more and got the filter for algae but reading the reviews I decided to get the plain filters to save money, big mistake, within a couple of days our tank was a mess with algae and discoloration and odor I never had that problem with these filters I recommend them 100%, I have a very large red earred slider and it is hard to keep his tank clear, but not with these filters I will never buy the plain filters again these get the job done!

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  • 1 out of 5 stars

    these do not work

    By tortuga on Jul 3, 2017

    The carbon filter cartridges work. These do not. I can do a 60% water replacement and the water is murky and filled with fine particulates the day after installing these with new sponges. The carbon filters work. These do not.

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  • 4 out of 5 stars

    Fluval filter

    By Mimi on Nov 21, 2016

    This product does it's job well and my tank benefits from it.

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  • 5 out of 5 stars

    good product

    By xxLauraxx on Sep 19, 2016

    Good product and I like ordering from chewy. Good prices and fast delivery.

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  • 1 out of 5 stars


    By Bethany on Aug 25, 2016

    To be fair I thought I was ordering the carbon filters, but didn't pay close enough attention. However, I gave these a try and found them to be next to useless. The water is consistently cloudy. The description says they will collect fine particulates, but this is simply not true. I absolutely love my filter and other filter pads that are available, but will never order these again. Blessedly this was an inexpensive mistake.

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