Undergravel Filter, what is it?

An undergravel filter is an aquarium filtration system installed beneath the aquarium substrate, usually gravel. It uses a perforated plate that creates a small chamber between the bottom of the tank and the gravel layer. Water moves through the substrate, allowing the gravel to function as a large biological filtration medium. Beneficial nitrifying bacteria colonise the gravel and process dissolved waste produced by fish, uneaten food and organic matter. Water movement normally comes from air lift tubes, an air pump or a compatible powerhead. Undergravel filtration offers a simple way to support water circulation, biological activity and aquarium stability without placing a large filter unit inside the visible part of the tank.

How does an undergravel filter work in an aquarium?

An undergravel filtration system works by drawing aquarium water through a layer of gravel and into the space beneath a perforated filter plate. From there, the system directs the water upward through one or more lift tubes. Traditional designs use an air pump connected to an air stone inside each tube. Rising air bubbles move water upward and create a gentle suction beneath the plate. Some aquarists instead connect a powerhead to increase the rate of circulation. In both arrangements, water continually passes through the gravel, exposing dissolved compounds to a very large surface populated by beneficial bacteria.

The gravel therefore serves as much more than decoration. Every suitable piece of aquarium gravel provides microscopic surfaces where bacterial colonies can develop. These microorganisms form an important part of the aquarium nitrogen cycle. Fish release ammonia through respiration and waste, while decomposing food and organic debris contribute additional ammonia. Bacteria convert this potentially harmful compound into nitrite, and another group of bacteria converts nitrite into nitrate. A mature biological filter therefore helps maintain safer water conditions for aquarium inhabitants.

Flow rate influences how effectively this process operates. For example, consider a 100 litre aquarium with a filtration flow of 200 litres per hour. The theoretical turnover equals 200 ÷ 100 = 2 aquarium volumes per hour. Increasing flow to 400 litres per hour produces a theoretical turnover of 400 ÷ 100 = 4 volumes per hour. Actual water movement through the substrate may differ because gravel depth, grain size, accumulated debris and equipment resistance affect circulation. Excessively fine material can restrict water movement, while very coarse material may allow organic debris to settle deeply between particles.

• Filter plates create the water chamber beneath the substrate.
• Lift tubes return filtered water to the aquarium.
• Gravel substrate supplies a broad surface for bacterial colonisation.
• Water flow transports oxygen and dissolved waste through the biological filter.
• Nitrifying bacteria convert ammonia and nitrite into less immediately toxic nitrate.

Oxygen availability also matters because the bacteria involved in conventional nitrification require oxygen rich conditions. Continuous circulation brings oxygenated water into contact with bacterial colonies throughout the gravel bed. This makes an undergravel system particularly suitable for aquariums where biological filtration forms the main objective. The design remains mechanically simple, but its performance depends heavily on maintaining suitable substrate circulation. If excessive detritus accumulates beneath or within the gravel, water channels may develop and some sections can receive less flow than others. Regular substrate maintenance therefore supports consistent filtration across the entire filter plate.

What substrate works best with an undergravel filter?

The choice of substrate has a major influence on the operation of an undergravel filter. Traditional systems work most effectively with medium sized aquarium gravel because water can pass between individual particles without moving the substrate itself. Very fine sand usually creates too much resistance and may fall through openings in the filter plate. Extremely large stones provide generous spaces for water movement, but those same spaces can trap uneaten food, faeces and other organic waste. A moderately sized, relatively uniform gravel bed normally provides the most practical balance between circulation, bacterial surface area and routine cleaning.

Substrate depth also affects the system. A layer that measures approximately 5 centimetres across a tank measuring 80 centimetres by 35 centimetres occupies around 80 × 35 × 5 = 14,000 cubic centimetres, which equals approximately 14 litres of substrate volume. This illustrates how much biological surface can exist inside even a relatively modest aquarium. Increasing the depth creates more potential bacterial habitat, but it also increases resistance to water movement. The aquarist should therefore avoid creating an unnecessarily deep gravel bed above the plate.

Unlike some modern aquarium substrates, specialised plant soils can create difficulties with this filtration method. Many aquarium soil products contain fine particles and nutrients that manufacturers intend for planted tanks. Continuous suction through these materials may move particles, alter nutrient distribution or cause compacted areas. Aquariums designed around dense root feeding plants often use different filtration arrangements that leave the substrate comparatively undisturbed. An undergravel system works more naturally with conventional inert gravel, particularly in community tanks, breeding systems and aquariums where durability and straightforward maintenance matter more than specialised planted tank substrates.

• Choose medium aquarium gravel that allows consistent water passage.
• Avoid very fine aquarium sand unless the particular filter design specifically supports it.
• Keep the gravel layer reasonably uniform across the filter plate.
• Prevent excessive accumulation of detritus between gravel particles.
• Check that decorations do not obstruct too much of the filter surface.

Large rocks, caves and ornaments also influence filtration because they reduce the area through which water can enter the substrate. A single ornament covering a small section rarely creates a significant problem, but several broad decorations can greatly reduce the effective filtration area. If a filter plate covers 2,800 square centimetres of the tank floor and decorations cover 700 square centimetres, approximately 25% of the available surface no longer receives the same direct flow. Thoughtful aquascaping can therefore improve the uniformity of water movement through gravel. Plants with extensive root systems require similar consideration because dense roots may alter local circulation over time.

Undergravel filter maintenance and aquarium cleaning

Regular undergravel filter maintenance focuses primarily on removing debris from the substrate while preserving the established biological community. As aquarium water passes through gravel, suspended particles and organic material can settle between individual stones. Some of this matter decomposes naturally, but excessive accumulation can reduce flow and increase the amount of dissolved waste in the aquarium. A gravel vacuum offers the most practical cleaning method because it removes loose debris while leaving most of the substrate and its bacterial colonies inside the tank.

Cleaning does not require removing and washing the entire gravel bed. In an established aquarium, aggressive cleaning can remove large populations of beneficial microorganisms and disturb biological stability. Instead, aquarists can clean sections during normal water changes. For example, if the gravel surface covers 3,000 square centimetres, cleaning approximately one third during each maintenance session means treating around 1,000 square centimetres at a time. This method removes accumulated material progressively while leaving much of the established biofilter undisturbed.

The lift tubes also require occasional inspection. Algae, mineral deposits and organic material can reduce the internal diameter of a tube and consequently lower water flow. Air stones gradually become clogged as well, which can reduce bubble production and weaken circulation. Replacing or cleaning an air stone restores more consistent movement. When the system uses a powerhead, the impeller and intake require routine maintenance according to the equipment manufacturer’s recommendations. Stable circulation matters because bacteria within the gravel depend on a continuing supply of oxygenated water.

• Use a gravel cleaner during scheduled water changes.
• Clean different areas of the substrate gradually instead of disturbing the entire bed at once.
• Inspect air stones for reduced bubble output.
• Keep lift tubes free from algae and deposits.
• Monitor the aquarium for changes in water quality and circulation.
• Remove excessive uneaten food before it enters the gravel bed.

Stocking level and feeding practices also influence maintenance frequency. An aquarium containing numerous fish produces more waste than a lightly stocked tank of the same volume. Likewise, excessive feeding increases the quantity of organic matter entering the substrate. If ten fish receive more food than they consume, the remaining particles gradually settle into the gravel and add unnecessary pressure to the filtration system. Careful feeding therefore reduces maintenance requirements and supports more predictable aquarium water quality.

An undergravel filter can remain functional for many years when the gravel stays reasonably clean and circulation remains consistent. Its simple construction contains few mechanical parts inside the aquarium, while the broad substrate area supports extensive bacterial colonisation. However, the system relies on the aquarist maintaining open spaces between gravel particles. Routine vacuuming, sensible stocking, controlled feeding and regular checks of the air pump or powerhead help preserve reliable biological filtration without frequently dismantling the aquarium.