Under Gravel Filter – What Is It & How Does It Work?

The under gravel filter is a filtration system in which gravel is used as a filter.

The gravel can be a good filtration material, especially if there is a substantial layer, more than 5 cm in height, of gravel.

The gravel provides mechanical and biological filtration if the aquarium is well-cycled.

What is an under gravel filter and how does it work?

undergravel filter

The under gravel filter consists of a rectangular grid (most cases plastic, but can also be made of conjoined pipes with holes) connected with a pipe for water circulation.

The under gravel filter is placed at the bottom of the fish tank, right on the tank’s glass, and then the gravel is placed on top of the  filter.

The under gravel filter uses the gravel as filtration media, thus providing mechanical and biological filtration since the beneficial bacteria usually makes colonies on objects, instead of just floating around in the water like the malicious types; the gravel is perfect for vast colonies of beneficial bacteria.

The filtration is made by circulating the water from the under gravel filter back into the aquarium.

The under gravel filter has tiny holes through which water progresses into the filter and uses the air or water pump to force the water up and return it back into the fish tank.

The other end can be submerged if needed. In this way, the water goes through the “purifying” bacteria that converts ammonia and nitrites to nitrates, which are less harmful.

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The working mechanism

The under gravel filters are usually made to be used with air pumps. Since air is lighter than water, it pushes the water up the filter pipe and back into the aquarium.

In order for this to work properly, the air tubing should be placed down the pipe to create water flow.

The use of an air pump is more effective on smaller tanks as opposed to large ones.

The under gravel filter is sometimes used with a water pump (power head), which when correctly placed in the upper end of the pipe of the under gravel filter,  a bigger water flow is created, thus, more effective filtration.

A water pump is also needed when an under gravel filter is used in an outside pond.

Since the pond is very big compared to the home aquarium, a strong power head is used to make current of water that goes through the gravel.

When you decide to use under gravel filter for your koi pond, an additional retaining wall should be underwater so the gravel keeps its shape and height in order to be a good filtration material.

The setup is the same: first, you place down the under gravel filter grid, then you put the gravel on it.

When everything is connected and the water pump is turned on, the water is drawn towards the gravel, sucked through the small holes of the under gravel filter grid and returned.

By cycling the water through gravel that has colonies of beneficial bacteria, you are accomplishing mechanical and biological filtration.

When your filtration material becomes dirty

The gravel is playing the part of the filtration material and, as you know, that material should be cleaned from time to time like any sort of filtration material.

The best way to clean your gravel is by using a gravel cleaner.

By siphoning the gravel, all the debris and organic waste build up goes down the drain into a bucket.

Once the gravel is clean,  your filtration material is ready to be used again.

When to consider an under gravel filter

under gravel filter

The under gravel filter is not as effective as other filtration systems when used as a stand-alone filter.

It might be useful enough for smaller-sized fish tanks, but for bigger tanks it may be used as an additional filtration.

Also note that the under gravel filter does not provide chemical filtration, which is a very important part of the filtration process.

Thus, in order to have chemical filtration in your tank, you would need additional items.

If your under gravel filter works accordingly, the water is clear and the fish look healthy and happy.

However, if the water starts to develop color and the fish start to become diseased, you might consider additional filtration sources, especially chemical filtration.


For smaller-sized fish tanks, an under gravel filter might work for you just fine, but if you have a large tank, then using an under gravel filter may be an add-on to your main filtration.

If you’ve decided on getting an under gravel filter, below are some of my recommendation:

7 thoughts on “Under Gravel Filter – What Is It & How Does It Work?”

  1. Hi there,

    Great website. Stumbled upon it while researching plants for beginners. I’m starting a small tank (either 5-10) which will have a small traditional water filter and am looking to also add an under gravel filter. I would like to know your opinion on how having an under gravel filter may affect plants. Would you suggest pairing the two?

    I’ve had a large tank for many years in the past but never explored live plants.


  2. I need to know if the little cartridges can be submerged under water. The part that the bubbles come out of? Plz help

  3. i was given a tank that had UGS i and a floval system to it

    i discharged filter pump in to UGS with airator pump to extract other end .
    used floval mechanical sys tem to supply pump
    water seldom had issues
    should i use a bio matt substrate under UGS to create buffer of the good stuff

  4. If I put a filter pump inlet at one end of fish tank under the gravel and outlet under the gravel at other end of fish tank would it work, I have the trays for under the gravel


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