What is an Under-Gravel Aquarium Filter?

An under-gravel filter is pretty much what the name implies, a filter beneath your aquarium gravel or substrate. It is made up of a perforated or slotted sheet of plastic, slightly raised above the base glass to allow water flow downward, through the gravel and up again through “uplift tubes”.  

Early on in the hobby, this was the filtration system. It doesn’t take much to run, maintenance is easy, it doesn’t clutter up the tank and for the most part – it does a very good job of keeping the water not only crystal clear but more importantly, safe for your fish to live in and thrive.

How does it work?

Under-gravel filters are usually run by air. Your aquarium air pump is attached via airline tubing to the base of the uplift tube or where the tubes are wide enough an air stone / bubble stone can be placed in the bottom of the uplift tube. The rising bubbles pull water upward causing low pressure below the filter plate, which in turn pulls water downward, through the gravel, through the slots and out the uplift tube/s. For increased flow rate it is also possible to place power head pumps at the top of the uplift tubes (remove the bubble option first).This will supercharge the flow rate but will also increase lateral / horizontal flow in the aquarium.

It is important to note here, that this system turns your aquarium gravel into a biological filter. Neither the plate, nor the uplift actually do very much toward the filtration process, besides facilitating the flow of water through the gravel – It is now your actual substrate that has become both mechanical and biological filter media. This is where the filtration takes place. Right there in your substrate.

When your under-gravel filter is working properly, two types of bacteria are present on every available surface, most importantly in your substrate. As much as the downward flow through the gravel, filters out particles from the water, these bacteria filter out dissolved compounds that are harmful to your fish. These compounds are Ammonia (NH3) and Nitrites (NO2).

These bacteria are Nitrosomonas and Nitrobacter. Nitrosomonas oxidises ammonia (NH3) – a dangerous compound of Nitrogen and Hydrogen and a waste waste product – into Nitrites (NO2) as a metabolic process. In turn, Nitrobacter oxidises Nitrites (NO2) into the much less harmful Nitrates (NO3). At this point, plants can make use of the available Nitrogen and what is left we dilute through regular water changes – a standard and necessary practice to keep your aquarium in peak health.

I have included a link below, where you can read a little more on this section of the Nitrogen Cycle as it pertains to your aquarium.


What are the advantages of an under-gravel filter?

NOTE: When discussing “advantages” it is often a point of your own perspective. As such, the advantages listed here are my own opinion. You may view these in a different light and I respect that. Please also note that improper construction, installation, use or maintenance of your under-gravel filter can dramatically reduce its effectiveness and these listed advantages are based on your filter running well. Maintenance is covered later.

An under-gravel filter, in my opinion has the following points in its favour.

  • It is unobtrusive and not likely to spoil the aesthetics of your aquarium layout
  • Installation is really easy and doesn’t depend on tank decor
  • It takes very little to operate, even a smaller air pump will do (depends on tank height)
  • Swimming space (fish) or growing space (plants) are basically maximised
  • Waste is pulled downward instead of floating around the tank
  • Flow rate is fully controllable and for the most part silent (depends on setup)
  • It is very low maintenance
  • It has a far larger surface area for bacteria than most other filter systems
  • Substrate’s role in filtration is improved, using the entire depth and not just the surface

What are the disadvantages of an under-gravel filter?

Again, these are my opinion and may be different from your perspective.

  • It does not work with soil substrate. If substrate is too fine there will be no water flow
  • Many plants will not thrive and some will even die with too much root-level circulation
  • Uplift tubes tend to discolour and collect slime (easy enough to clean though)
  • The slots in the base plate can become blocked over time (maintenance is key)
  • Often base plates are not available in sizes that match the aquarium being set up
  • Food particles may be drawn into the substrate out of the reach of lower level fish
  • Slime can collect below the base plate impeding the flow of water.
  • Cannot easily be removed if tank requires anti-bacterial treatment
  • Must be installed when tank is completely empty during initial setup

How do I install my under-gravel filter?

Firstly, examine the base plate you have. Make sure all the slots are unobstructed. These sometimes do not form properly during the moulding process but can easily be opened by carefully cutting the obstructing plastic away with a sharp blade. If it is a previously used filter, make sure that it is thoroughly scrubbed clean; you won’t easily get another chance to do this. Rinse well so that no bits of cut plastic or any foreign matter is on the plate and nothing is blocking the holes.

Secondly, the base plate has to be placed directly onto the bottom of the aquarium, so if you are setting up your tank after dismantling a previous setup or if you are starting afresh with a newly obtained aquarium; the under-gravel base plate is the first thing to go in. As far as possible it needs to cover the entire floor of the tank so if you have bought one slightly bigger it can be cut to fit but if it is a little smaller than the floor of the tank it is best situating it against the back panel and centrally between the two ends. Remember, dirt will collect in the substrate in any area not covered by base plate so the tightest fit you can manage is desired. A DIY alternative will be discussed later that allows you to make a base plate any size you like to fit the aquarium you have if you cannot find a suitable one at your local fish store (LFS)

Once you have placed the base plate, hollow side down directly onto the clean floor of the tank you will need to ensure your uplift tubes are clear of debris and installed fully into the base plate holes at each end. If the fit is not tight, the tubes could come loose and substrate will fall in impeding the flow of water even if you put the tube back so make sure they cannot easily be bumped loose. If you have to glue them in, super glue or silicone is recommended. If you use silicone it needs 24hrs to fully dry before water can be added. Once your uplifts are secure, attach one end of airline tubing to the inlet on the base plate next to the uplift tube or if using bubble stones thread them and their attached airline down the uplift to the bottom. If you are using power heads to move the water through the system you can add them now or after filling the tank but you must make sure they fit well and are secure so as not to fall off the uplifts which could potentially cause damage to fish, plants or even the tank glass.

Thirdly, add your substrate / gravel. Consider the fish you intend to keep… if they are known diggers or substrate movers, it is advisable to place filter mat on top of the base plate to prevent it from becoming completely exposed by the digging or shifting of substrate by the fish. Gently add your rinsed (to remove dust) substrate / gravel so as not to damage the base plate. If you have some substrate in an established aquarium, you can add some of it now to “seed” and accelerate bacterial growth on the new substrate. Do not install rocks or other heavy decor directly onto the base plate. The entire surface of the base plate must be covered with substrate preferably not smaller than about 2-3mm. The gravel should be about 5 cm deep against the front glass and sloped gently up to the back where between 7 -10cm works well.

Once your under-gravel filter is correctly installed and connected to air pump through a check valve you are ready to continue. Place whatever decor you are using on top of the gravel and plant any plants you are using about a third way into the gravel depth. Roots will eventually grow down to the filter and that is unavoidable but best not to place them too close to it to begin with. Remember many plants do not appreciate water movement through their root systems so do a little research to avoid wasting money and destroying perfectly good plants by providing conditions they cannot adapt to.

Once you have decorated and planted, gently fill the tank with clean, de-chlorinated tap water or rain water, if you have and turn on the air pump or power heads. Your under-gravel filter can now begin cycling (Growing the necessary bacterial colony that will ensure a healthy aquarium)

Depending on whether or not you used established substrate to seed your filter you should leave the tank like this for between 2 -4 weeks. In my opinion 2 weeks is the minimum you should wait before adding fish to the aquarium. This allows the bacterial colony time to fully establish itself and begin properly processing any decaying waste in the water. It is also a good time for you to decide whether you like everything the way it looks and to decide on and source the fish you want to keep. Some really hardy species can be added after the first week. I don’t suggest you do this without having at the very least observed a marked improvement in water clarity from initial setup. If you can still see any discolouration in the water you should avoid adding anything. The same applies if you are going to be changing anything significant so as not to stress your fish out fiddling too much right after adding them. Stress will lower fish’s immunity and they will not be at their best which is bad for them and discouraging to see in a new tank. A stressed fish is also more prone to contracting diseases.

How do I maintain my under-gravel filter system?

Never over feed your fish. Uneaten food will rot. This applies to all aquaria.

The number one reason any filtration system may seem inadequate is over feeding. More food means more waste, not only from faeces and urea but plain simple uneaten food rotting in the water. Avoid this from the start by only feeding twice a day and not more than the fish can consume in 5 minutes or less. If they are not eating it all that day, net out any excess before it has time to disintegrate.

Maintaining your under-gravel aquarium filter system is vital to keep it performing as it should and to prevent the build-up of sludge. Remember, your gravel is now also your filter media.

 As with all filters, the filter media needs to be rinsed periodically. As with any aquarium, the substrate on top of your under-gravel filter will accumulate dirt. Because it is now your filter media it needs more regular cleaning.

 The easiest and most effective way to do this is to vacuum it with an aquarium gravel vacuum. Do this directly to a bucket and use the dirt and water together on your outdoor plants. They will benefit from all the nutrients while your filter stays clear of excess sludge and remains as effective as ever. Do this when you are draining part of the tank anyway for a water change, then nothing is wasted. Be careful not to drive the gravel-vac too deep into the substrate as you could damage the base plate. Be careful near plants as roots could be damaged.

Once a month and at minimum once every 3 months attach your siphon tube to the uplifts with air turned off, to remove any sludge that may have collected beneath the base plate. If you are using power heads, this will be less important than vacuuming the gravel to prevent the power heads from redistributing the sludge around the tank. In the case of power heads keep your gravel especially clean with more regular vacuuming.

A video showing some UG filter options and some tips on maintenance.


Make your own Under-Gravel Aquarium Filter

If you are keen to use an under-gravel filter in your next aquarium, it is possible you will not find one that fits right so I am including a link to a video on how to DIY a simple one to fit your needs.

You may have to buy the odd item but could already have everything you need laying around at home. (haven’t yet found a simple and straightforward video, all guys tend to ramble a lot)

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