You and Your Under-Gravel Aquarium Filter

By: Paul Barry-Cox

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)

Planted Tank: Hi/Low Tech?

With the increasing popularity of planted tanks and aquascaping, a few people are asking questions about High Tech or Low Tech setups. In this post I will try to answer a few questions on the subject, without getting too technical (no pun intended)…

What is the difference? High Tech setups uses CO2 injection, an intensive fertilizing schedule and specialized high intensity lighting. Low(er) Tech setups run on standard lighting, the occasional addition of fertilizers and mostly no additional CO2

Do I need to have a High Tech setup? The short answer is “No”. It really all depends on the difficulty factor of the plants you are attempting to grow, and to a large extent the size of your budget and the time you can dedicate to your tanks. Stunning low tech planted tanks can be found everywhere. All planted tanks (all aquariums really) demand maintenance in various degrees and low tech planted tanks are no exception. High tech setups requires a much more dedicated approach though.

Going ‘high tech’ generally implies an interest to grow more types of, and more demanding plants, and to grow them in a more optimal way to produce colors and density greater than what is possible without CO2 injection.
Dennis WongThe 2HR Aquarist

Do my plants need CO2? Yes. All plants need CO2. In the normal aquarium with a few normal plants, the water should contain enough dissolved CO2 to sustain growth. Keep in mind that fish exhale CO2 into the water as well. However, when plant density or growth reaches higher levels, there’s a need to supplement CO2. This can be done in more that one way. The addition of Liquid Carbon additives supply in this need to some extent, and still qualifies as “Low Tech“. Certain more difficult or demanding plants will not do well or may be downright impossible to grow without injected CO2. Injected CO2 will immediately put your tank in the High Tech category, and this is where the fun starts.

Can I upgrade to High Tech in stages? Not really… The problem is that one High Tech item without the others, will usually create an imbalance that will cause other less desirable issues. Algae growth being the biggest culprit. Plants growing with injected CO2 will use up nutrients faster and will also demand more light for photosynthesis. Absence of any of these will allow the opportunistic growth of algae as one effect, and also cause deterioration of your plants due to lack of either nutrients or light, or both. The keyword here is “balance”. The three components of a High Tech setup (CO2, Lighting and Nutrients) needs to be in balance to be effective.

What is this “Pearling” that everyone is talking about? During the process of photosynthesis plants use CO2 and “exhales” oxygen. This oxygen gets released and dissolves into the water column. This is actually very good for your livestock, supplying them with much needed oxygen. As the dissolved oxygen in the tank reaches saturation level, the oxygen that is released by the plants cannot dissolve in the water anymore and floats to the surface as bubbles. This is actually quite a good indication of two things; First, that the oxygen levels in your water is sufficient, and secondly, that your plants are doing well.

Future posts will discuss all three the basic elements of a High Tech setup in more specific detail.

What is Aquascaping – Part 1

To know Mother Nature, is to love her smallest creations’

Takashi Amano

I often hear people ask what the difference is between a Planted Tank and Aquascape. Although it may sound like a straightforward question and an easy one to answer, there is quite a bit more to it than that.

Wikipedia “…the craft of arranging aquatic plants, as well as rocks, stones, cave work, or driftwood, in an aesthetically pleasing manner within an aquarium—in effect, gardening under water.”

It does not stop there, however… Aquascape designs include a number of distinct styles, including the garden-like Dutch style and the Japanese-inspired nature style.[1] Typically, an aquascape houses fish as well as plants, although it is possible to create an aquascape with plants only, or with rockwork or other hardscape and no plants. 

One can easily make the mistake of assuming that any tank containing plants and/or hardscape, arranged in a pleasing way, can be labelled as an Aquascape. This definition could be considered as acceptable by some, but the technical truth is that, in theory, a “proper” Aquascape must conform to a specific style in order to be classed as a true Aquascape.

There is currently a few accepted styles for Aquascaping that can be adopted, namely Dutch, Nature, Iwagumi, Jungle, biotope, Paludariums and  Saltwater Reefs. To create a better understanding of the difference between these styles, let’s look at each one in more detail. (All of these styles will be discussed in dedicated posts in the near future.)

Dutch Style

With its origins in the 1930’s, the Dutch Style is certainly the oldest aquascaping style that is still popular. The Dutch Aquarium style does not rely on the use of wood, rocks and other hardscape materials to create impact. The main stars of the show are the aquatic plants in intricate placements and groupings, to help create in-depth perspective within the aquarium by complimenting each other in shape and color.

Most magnificent Dutch planted aquariums are characterized by high density, rich contrast and subtle use of color and texture, with the planting often on terraces to create different levels.

The most important requirement for aquascapers who want to approach the Dutch Aquarium style is to have extensive knowledge regarding aquatic plants.

Nature Style

Introduced by Japanese aquarist Takashi Amano back in the 1990’s and becoming one of the dominating aquascape styles since then, the Nature Style is in essence a representation of particular terrestrial landscapes.

The careful use of rocks, driftwood and plants, arranged in order to create natural ambience and flow, creates a miniature landscape that can be harmoniously intriguing as well as astonishingly beautiful.

The Nature style has three basic sub-styles: concave, convex and triangular. These will be discussed in more detail in future posts.

Iwagumi Style

The Iwagumi aquascaping style is a subset of Japanese Nature Aquascaping that requires a significant amount of experience to implement and maintain.

The Iwagumi style follows a general layout that requires a balance between open space, hardscape, and scale between each aspect of the design.

“Iwagumi” in Japanese translates to “Rock Formation,” which in turn refers to the stone architecture, formations, and placement defining the features of the design. Essentially, the stones in an Iwagumi aquascape act as the structure of the entire design.

Traditionally, there are three stones in an Iwagumi aquascape; however, the aquascaper has the liberty to build his or her design based upon preference.

Part 2 will deal with some other styles, namely Jungle, Biotope, Paludarium and Saltwater Reef.