How Does An Undergravel Filter Work?


Any aquarium needs a filter to provide the best habitat for the fish. That is why choosing a quality filter that works effectively is extremely important. When it comes to this, there have been many highly recommended Undergravel Filters for their aquarium.

So, what do we need to know about this product? How does an Undergravel filter work? Find out below!

What Is The Undergravel Filter Product? 

Undergravel Filters, also known as UGFs for short, are mechanical and biological filters used for aquariums. The filter is equipped with a mesh layer and stone support to filter the water.

Specifically, the pebbles inside the function as a mechanical filter that helps collect debris. With the help of oxygen-rich water and bacteria growth, the biological filter will work.

If you use a UGF filter, be careful not to use sand as a substrate inside the tank because the sand is very small and easily falls into the filter, leading to clogging or the filter not working properly.

How Does An Undergravel Filter Work?

The working mechanism of the Undergravel Filter is quite simple, and you can easily install it inside your tank. A mesh covers the gravel inside the filter to prevent the gravel from approaching the bottom of the aquarium, and at the same time, water can flow through the gravel.

The upstream part is equipped with a water pump, and then the water is drawn up and flows through the stone. This mechanism works like a mechanical and biological filter. There is also another air pump to help draw water through the gravel.

Next, the water tube and powerhead will help pump the filtered clean water back into the tank. At the same time, the air pump also helps push air bubbles at the bottom of the tank into the lift pipe to help push clean/fresh water to the top of the tank.

The whole filtration process can be understood as a stone layer that helps filter the water, and then the water pipe will return the clean water sources to the top of the tank.

Refer to more details of the forms when using an undergravel filter for the water purification process below:

Chemical Filter

The filter also possesses substances such as activated carbon to absorb the waste from food, sludge of moss, fish stool, and other impurities in the lake.

You should note that the undergravel filters will have many types, and several products will not support the chemical filter system.

The canister filter will be a great solution if your filtered product lacks a chemical filtration system. This filter also has a very effective chemical filtration function, filtering impurities and other dirt.

Biological filter

When an undergravel filter is used, the stone at the bottom of the filter and the walls of the filter tube will initiate biological filtration on its own.

The biological filtration process promotes the growth of bacteria to help remove unwanted substances from the water and create a new stream of water that is safer for fish to live in.

Mechanical filter

Undergravel filters possess stone particles that are both aesthetic for the filter and mechanically filter water.

Specifically, the mechanical filter will operate when water flows through and traps large particles. Note that the filter needs to be carefully removed when you need to clean or change the water in your aquarium.

What Are The Undergravel Filter Installation Steps?

Method: Sand Cap

Refer to the step by step of sand cap method right below:

  • Step 1

First, you can put the parts of the undergravel filter on the lower surface of the aquarium. Note: You need to clean the aquarium before installing the Undergravel Filter system.

You need to install air pumps in conjunction with airline tubing so that airstones operate. Additionally, you can use a double outlet air pump by just one ban. You can easily turn it off/on to operate it.

  • Step 2

You need to equip a piece of poly fiber suitable for the filter plate surface. The addition of this poly-fiber class aims to prevent debris and sands that can be flooded inside.

Thin poly-fiber pieces are prioritized for use because of their great features. You should not use cotton fabrics because they will be corrosive, damaged, and efficient filtration is not high.

  • Step 3

You need to pour a stone layer (1 / 2-1 “) and pre-soaked Seachem Matrix on poly fiber. Not only that, some hard fish species such as cichlids can also pour a layer of crushed corals. This factor aims to support good traffic and provide many surfaces for living fish.

In addition to equipping porous substrates such as crushed corals, you can also choose an area for denitrification. You can also put an additional gravel or grid layer to prevent it with sand if you want.

  • Step 4

Finally, you can pour a layer of sand of 1.5-2 “on the stone section. If you have a species of digging fish, it is necessary to pour 2 ‘to limit the digging fish to disturb the stone layer below.

Method: Reverse Flow

Follow the steps below:

  • Step 1

It would help if you placed the filter element on the bottom of the tank, keeping in mind that the tank needs to be cleaned before installing the filter. With this method no need for airstones.

  • Step 2

Pour porous substrates such as Hydroton or Seachem Matrix and gravel on top. If you do not have these substrates, you can replace them with larger river gravel. However, this type of gravel has many limitations on water movement and does not support nitrate reduction.

  • Step 3

For water to flow down the absorber, you need to use a submersible powerhead. This element helps to reverse the flow and cause the water to rise.

To keep it mulm-buildup free, you need to pump water through the substrate, which will also cause water quality deterioration.

  • Step 4

With this filter, you are required to install a pre-filter above the powerhead. This element helps to prevent debris from being pushed by the powerhead at the bottom. Usually, inferior quality filters will not have this element, and it often causes many problems.


You must have understood the importance of a quality filter through the above information, especially the Undergravel Filter. Always consider carefully choosing a good filter to keep your fish as healthy and clean as possible.

After learning “How does an Undergravel filter work?”, you can immediately apply it to your installation process. Hope this information is helpful!

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