What is a Fluidized Bed Filter?

Fluidized Bed Filter


A fluidized bed filter is a container that hangs on the back of your fish tank.  It can also be a cylinder or even a canister which can sit in a sump on the floor beside or under your fish tank.  This will be connected to a pump to push water through the filter.

These filters are actually known by many terms.  If you are searching for one then try one or all of the following terms to get the best results:


fluidized bed filter, phosphate reactor, media reactor, aquarium reactor, phosban reactor, co2 reactor, biopellet reactor, carbon reactor, calcium reactor, chemical reactor

Many people also think of these filters as a “suspended particulate filter” or a “suspended sand filter,” for obvious reasons as these phrases describe them.


In very basic terms these filters, connected to pumps, force water through a load of small granules – usually sand, specially designed plastic or silica chips.

Aquarium fluidized bed filters are an ideal addition to large or heavily stocked aquariums.  By “fluidizing” or suspending fine grained media in a column of water, they increase surface area and contact time for highly efficient biological filtration.  Aquarium fluidized bed filters are ideal for planted aquariums, as they won’t wear off your CO2.

The use of fluidized sand filters or multi-purpose reactors for aerobic bio-filtration is no longer a “new” concept. However, the use of these filters are still not well known to many aquarium keepers.

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How do Fluidized Bed Filter Reactors Work?

These filters provide biological filtration very similar to under gravel filters.  Nitrifying bacteria colonies grow on the surface of the particles of sand in the bed, and process ammonia and nitrite into nitrate as the water passes through the filter.

These aquarium filters usually do not come with an integral pump, so a separate water pump will need to be used.

The water is pumped into the bottom of the filter up through the media. Then it flows back into the tank from the top of the filter body. Because of the speed at which water is passed through the filter media, it becomes ‘fluidized’ – the sand grains (or pellets or other granules) are constantly being pushed up by the current and then falling back through the water in response to gravity.

You might like the look of the media swirling around the reactor, people have been known to buy one that hangs on the side of the tank for added attraction.

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Fluidized bed filters do not come with all of the hoses, pump, connectors, well-formed intake and return tubes, zip ties and hose clamps that you may need. This can make these filters a little more difficult to set up for a beginning aquarist.  It is important to check what is included with the filter and then consider what other parts you will need to suit the location of your tank and where you will place the filter (and the pump).  We recommend that you also buy some tubes that are soft and flexible for that difficult bend you may have to get around to make the best connection.

When you plan your plumbing it may be worth adding a good pre-filter on the intake and a ball valve coming off the pump before it inflows the filter.  This has proved very successful for many fluidised filter owners.

If you want to buy more in one package then there are options such as the AquaTop Media Reactor with Pump kit that come with Aquatop’s SWP-360 adjustable flow water pump and sediment discharge filter with Intake and output tubing included.



They are easy to maintain. You won’t disturb your bacterial bed, which is a problem with some biological filters. They can also, depending on the media used, treat contaminants such as salts, remaining chlorine, nitrogenous waste and heavy metals in the water.

Fluidized bed filter reactors require very little maintenance themselves.

However, the pump will require regular maintenance, including cleaning the impeller, impeller shaft, and intake and outlet as debris collects and algae and bacteria grow. If this is left unchecked, it causes reduced water flow, which leads to poor filtration.

You will also need to monitor the sand bed, as it can eventually settle and become stagnant. This should not happen for a long time, maybe even years, and is most likely to be a problem after an extended power outage.

Pros and Cons


  • Fluidized Bed Filters are cheap

  • They work very well once set up

  • The churning of the media in the filter can be very attractive – lots of videos available showing this on Amazon and YouTube.


  • However, they need plumbing with pumps usually not included.

  • The quantity of media included can be too low – read a few reviews for each product to check.
  • Some models – cheap means poor quality control. The cheaper ones can leak too!


If you want an overview of other types of filter check out our ‘Types of Filter‘ article HERE

9 thoughts on “What is a Fluidized Bed Filter?”

  1. The AquaMaxx BioMaxx BioPellet Reactor has been great for my fish tank, but I had to make sure I tightened every single connection…. I didn’t tighten it that much at first and that was a big mistake… it doesn’t come fully tightened out of the box.

  2. So glad you talked about fluidized bed filters. I finally get how they work with the nitrifying bacteria colonies helping to keep the nitrate levels at bay. Now I gotta look up the videos of the media swirling around the reactor!


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