Under Sand Filter Aquarium

An under sand filter (also called an undergravel filter or UGF) pulls water down through a sand or gravel substrate and through a perforated plate positioned beneath it, using lift tubes and either air pumps or powerheads to create flow. The substrate itself becomes biological filtration media. The filter is built into the bottom of the tank, invisible from the outside, and handles biological and some mechanical filtration in a single setup.

Whether this approach is worth using in your aquarium depends heavily on what you're keeping, how deep your substrate is, and how much maintenance you're willing to do. UGFs have real advantages in certain setups and serious drawbacks in others. I'll cover how the system actually works, which setups benefit from it, how to install and maintain one, and what the alternatives look like.

How an Undergravel Filter Actually Works

The perforated plate sits on the bare tank bottom and covers most or all of the floor area. Substrate (typically 2 to 3 inches of medium gravel or coarse sand) sits on top. Lift tubes rise from the plate at the corners or edges and extend above the substrate.

Airstone Drive vs. Powerhead Drive

In the most basic configuration, an airstone inside each lift tube creates rising air bubbles. The bubbles carry water upward with them, creating a flow that draws water down through the substrate and into the space under the plate, then up through the lift tubes. This is the classic air-driven undergravel filter.

The alternative is a powerhead mounted on top of the lift tube. Powerheads pull water up through the tube more forcefully than airlift, increasing flow rate and filtration efficiency. A Hydor Koralia Nano 240 or a basic Rio Mini 50 powerhead works for this on a 20-gallon tank.

The Reverse Flow Option

Some undergravel filters can be run in reverse flow mode: water is pushed down through the lift tubes and forced up through the substrate from below. This prevents debris from clogging the substrate and reduces channeling. It requires a powerhead. The biological filtration still happens in the substrate; the direction of flow just changes.

When an Under Sand Filter Makes Sense

UGFs work well in specific situations.

Heavily Planted Tanks (With Caution)

Some planted tank keepers use UGFs with fine-grained substrate because the flow through the root zone increases oxygenation and nutrient availability to plant roots. The tradeoff is that dense root systems can block flow over time, reducing filtration efficiency.

Breeding and Fry Tanks

A UGF with a fine mesh cover over the plate works well for breeding setups because there are no strong intakes that can trap small fry. The gentle draw through the substrate doesn't create the powerful suction that standard HOB or canister filter intakes do.

Low-Tech, Budget Setups

For a simple goldfish tank, betta tank, or classroom aquarium on a tight budget, a UGF with a basic air pump costs $10 to $20 total. It provides reasonable biological filtration without the complexity of a canister or HOB.

Fish That Prefer Substrate Activity

African cichlids, sand sifters, and species that actively move substrate can disturb and clog standard sponge filter bases or uproot plants. With a UGF, substrate activity doesn't compromise the filter's function.

For a broader comparison of filter types, the best aquarium equipment guide covers UGFs alongside HOBs and canisters in direct comparison.

Installation: Step by Step

1. Clean the tank thoroughly before adding any equipment.
2. Place the perforated plate on the bare tank bottom, covering as much floor area as possible. Use extension pieces if your tank is larger than a single plate covers.
3. Insert the lift tubes into the slots at the corners of the plate. Adjust height to sit just above your final substrate level.
4. Add substrate on top of the plate. Medium gravel (3 to 5mm grain size) works best for standard UGF use. Fine sand can clog the plate more easily unless you're using reverse flow. Depth should be at least 2 inches, ideally 3 inches, for adequate biological filtration capacity.
5. Attach airlift tubes to your air pump tubing or mount powerheads on top of the lift tubes.
6. Fill the tank slowly. Check that substrate is distributed evenly and that the plate is flat on the bottom.
7. Run the system for 4 to 6 weeks to cycle before adding fish. Test for ammonia, nitrite, and nitrate weekly.

Penn Plax makes a standard undergravel filter (model UGF10B for 10-gallon tanks, UGF55 for 55-gallon tanks) that includes plates, lift tubes, and basic instructions. It's available on Amazon for $10 to $20 depending on size.

Maintenance: The Part Most People Underestimate

The main maintenance task for a UGF is preventing the substrate from becoming clogged with debris. This is where the design has a significant drawback compared to external filters.

Use a gravel vacuum during every water change to siphon through the substrate. Don't just vacuum the surface; work the siphon down into the gravel to pull out trapped detritus. If you skip this for a few months, debris accumulates under the plate and begins decomposing anaerobically, releasing hydrogen sulfide gas, which is toxic to fish.

Plan for a deep clean every six to twelve months. This means removing fish to a temporary tank, pulling all the substrate out, cleaning the plate and lift tubes, and replacing at least half the substrate (keeping the other half to preserve beneficial bacteria colonies).

For a densely planted tank, this deep clean disrupts roots significantly. It's one of the reasons many planted tank keepers avoid UGFs despite their benefits for root-zone oxygenation.

For a look at filtration equipment that requires less substrate maintenance, the top aquarium equipment overview covers canister and HOB options with long-term maintenance schedules.

UGF vs. Other Filter Types: Real Comparisons

The UGF's weakest point is mechanical filtration. Solid waste doesn't get removed from the system; it gets pushed into the substrate where it breaks down. In a lightly stocked tank with infrequent feeding, this is manageable. In a heavily stocked tank, it leads to high nitrates and oxygen-depleted dead zones under the plate.

FAQ

Can I use fine sand instead of gravel with an undergravel filter? Yes, but it requires more care. Fine sand (under 1mm grain size) can compact and reduce flow through the plate. It also gets drawn into the lift tubes more easily. If you want to use sand, coarse sand (1 to 2mm) is more practical. Alternatively, use the reverse flow configuration, which pushes water up through the substrate from below and tends to work better with sand.

Does an undergravel filter replace the need for a regular filter? It can provide adequate biological filtration for lightly stocked tanks. However, it provides almost no mechanical filtration, meaning solid waste stays in the system rather than being removed. Most experienced aquarists run a UGF alongside a HOB or sponge filter, using the UGF for biological filtration and the secondary filter for mechanical media.

How deep should the substrate be over an undergravel filter? A minimum of 2 inches is needed for the beneficial bacteria to colonize adequately. Three inches is better, especially in larger tanks. Too little substrate means insufficient biological filtration capacity. Too much substrate increases flow resistance and can create anaerobic zones that defeat the purpose.

Why do some aquarists say UGFs are outdated? Modern HOB and canister filters provide better mechanical and biological filtration with less maintenance difficulty. The substrate cleaning requirement for UGFs is genuine labor that external filters don't need. For most tank setups today, a sponge filter or HOB is simpler and more effective. UGFs still have a place in specific use cases (breeding tanks, budget setups, cichlid tanks) but they're no longer the standard recommendation they were in the 1980s and 1990s.

Wrapping Up

An under sand filter works well when you choose the right tank for it. For budget setups, breeding tanks, or heavily stocked cichlid tanks where substrate activity is normal, it's a practical and inexpensive option. The maintenance requirement for regular substrate vacuuming is non-negotiable. Skip it and you'll end up with an anaerobic mess under the plate. Plan for deep cleans every six to twelve months, and run a secondary filter for mechanical filtration if your stocking level is more than light.