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What is An Undergravel Filter?

Undergravel filters are one of the oldest filtration methods, used successfully for years to decrease the buildup of harmful waste in the aquarium. This helps lower the stress of deep cleaning the tank.

A typical UG filter consists of perforated filter plates that lie on the bottom of the aquarium, beneath the gravel bed. At the back of the plates, a lift tube pulls air towards the bottom, creating a vacuum as air bubbles rise to the surface. This vacuum pulls water down through the gravel and the filter plate.

A more recent advancement is the power filter, which uses a powerhead water pump to mechanically pull water up the lift tubes. This method is more efficient than the vacuum generated by older air-pump-driven systems. In fact, these filters can be so effective that a separate canister filter may not be necessary.

How An Undergravel Filter Works

An undergravel filter (UGF) is a classic system that uses perforated filter plates placed beneath the gravel substrate. This design effectively turns the entire substrate bed into the primary medium for both mechanical and biological filtration.

The filter operates by pulling water down through the gravel via a lift tube, which is powered by an air pump or powerhead. The gravel first acts as a mechanical filter by trapping large debris. More importantly, its vast surface area hosts beneficial bacteria that perform biological filtration, converting toxic ammonia and nitrite into less harmful nitrate as part of the nitrogen cycle.

The Pros Of Undergravel Filters   

Despite newer technologies, UGFs offer distinct advantages that keep them relevant for many aquarium setups. Understanding these benefits is key to appreciating why this classic filtration method endures.

Simple To Install And Operate With No Moving Parts

UGFs feature a straightforward design with minimal components, making installation quick. The lack of complex moving parts ensures high reliability and ease of operation, often requiring little more than a basic setup.

UGFs Are Discreet And Improve Aquarium Aesthetics

The filter plates are hidden beneath the gravel, keeping all mechanical components out of sight. This design preserves the natural aesthetic of your aquarium, avoiding the visual clutter of external or bulky in-tank equipment.

Undergravel filters keep your aquarium looking natural and uncluttered.

Effective Biological Filtration With Ugfs

Undergravel filters excel at providing robust biological filtration. The large surface area of the gravel substrate creates an ideal habitat for beneficial aerobic bacteria, which are essential for maintaining a stable and healthy nitrogen cycle.

Inexpensive Acquisition And Operation Costs

UGFs are an attractive option for budget-conscious aquarists due to their low purchase price and minimal operating costs. They typically require only an inexpensive air pump, and the gravel itself serves as the filter media, eliminating the need for frequent replacements.

Versatile, Customizable, And Compatible Designs

Undergravel filters are highly versatile. They can be operated with a gentle air pump or upgraded with a powerhead for increased circulation. They are also compatible with supplementary chemical filtration, allowing them to be adapted to various tank needs.

Benefits Of Substrate Aeration

The constant water movement through the gravel bed aerates the substrate, discouraging the formation of anaerobic dead zones. This process promotes a healthy environment for beneficial bacteria and helps prevent the buildup of harmful gases like hydrogen sulfide.

The Cons Of An Undergravel Filter Setup

While they have their advantages, undergravel filters also come with several notable drawbacks. Aquarists should carefully consider these limitations before choosing this system for their tank.

Challenging Long-Term Maintenance For UGFs

Long-term maintenance can be demanding. Detritus and waste accumulate under the filter plates over time, requiring frequent and thorough gravel vacuuming to prevent clogs, system inefficiencies, and poor water quality.

Not Suitable For Diggers Or Specific Aquarium Setups

Undergravel filters are unsuitable for tanks with fish that dig, such as certain cichlids, as their burrowing can disrupt water flow and create anaerobic dead zones. They are also impractical for complex setups like reef tanks, where cleaning is difficult.

Potential For Noxious Gases And Dead Zones

Neglected maintenance can lead to anaerobic conditions within the substrate, producing noxious gases like hydrogen sulfide (identifiable by a "rotten egg" smell). Consistent gravel vacuuming is essential to prevent these harmful chemical buildups.

Neglected maintenance can create toxic dead zones in the substrate.

Risk Of Fish Getting Trapped Under Filter Plates

Smaller, bottom-dwelling fish like Kuhli loaches may occasionally get under the filter plates. While concerning, they often find their way out on their own. It is usually best to observe for a short period before dismantling the tank.

Undergravel Filters And Planted Tanks: A Potential Problem

The continuous water circulation through the substrate can disrupt nutrient uptake and root development for some rooted plants. This issue can be mitigated by using techniques like "blind potting" plants in separate containers to protect their root systems.

Possible Noise And Visual Disturbance

The air pump used in many UGF systems can be a source of noise and vibration. The resulting bubbles can also detract from the aquarium's peaceful aesthetic. Using an air stone can help disperse bubbles more finely, reducing noise and visual impact.

Limitations In Chemical Filtration

Undergravel filters lack comprehensive chemical filtration, which can lead to the accumulation of dissolved organic compounds and nitrates. This can be addressed by adding supplementary chemical media like activated carbon or zeolite in media bags on the filter plate.

Undergravel Filters Vs. Other Common Filtration Types

When choosing a filtration system, it's important to see how undergravel filters compare to modern alternatives like hang-on-back (HOB) or canister filters.

Choosing the right filter depends on your specific aquarium needs.

When To Consider An Undergravel Filter For Your Tank

An undergravel filter is well-suited for fish-only aquariums where robust biological filtration is a priority, and the tank does not contain burrowing species or extensive live plants. It can be an excellent, low-stress choice for first-time aquarists due to its simple setup and operation. For best results, use a coarser gravel layer beneath a finer one to optimize water flow and bacterial colonization.

A Guide To Selecting And Setting Up Undergravel Filters

Selecting and installing the right undergravel filter (UGF) is key to maintaining a stable and healthy aquarium. Proper consideration of your tank's specific needs ensures effective, long-term filtration.

How To Choose The Right Undergravel Filter

Careful planning is required to ensure your UGF matches your aquarium's demands.

• Tank Size: Larger tanks may need multiple filter plates and air pumps or powerheads to ensure adequate circulation across the entire substrate.

• Filtration Requirements: A higher bio-load, from more fish or messy eaters, demands a more robust setup, possibly with supplemental filtration.

• Additional Needs: Consider adding air stones to improve oxygenation or carbon inserts in the lift tubes to enhance chemical filtration.

• Budget: While UGFs are cost-effective, budget for potential additions like powerheads or chemical media to optimize performance.

Effective Setup And Use

Setting up an undergravel filter is a straightforward process for most aquarists.

1. Place the filter plates at the bottom of the aquarium, ensuring they cover the entire surface.

2. Add an appropriate layer of gravel substrate over the plates.

3. Attach an air pump or powerhead to the lift tube to initiate water circulation.

When used in outdoor ponds, UGFs require frequent manual removal of large debris like leaves to prevent clogging. A consistently running air pump is crucial to maintain water circulation and prevent stagnant spots.

Setting up an undergravel filter is a straightforward process.

Common Mistakes To Avoid With Undergravel Filters

To maximize the effectiveness of your undergravel filter, avoid these common pitfalls.

• Neglecting Maintenance: Irregular gravel vacuuming allows detritus to build up, leading to dead zones and the release of noxious gases.

• Using Incompatible Setups: Avoid using UGFs in tanks with active digging fish or densely rooted plants without proper adaptations like potting.

• Relying on UGFs Alone: UGFs are limited in chemical filtration. Supplementing with media like activated carbon will improve overall water quality.

How to Best Utilize an Undergravel Filter

If you plan on using an undergravel filter, keep the following tips in mind to ensure it runs effectively and efficiently in your aquarium setup.

• Use a coarse gravel substrate; avoid sand.

• Perform regular gravel vacuuming to remove waste.

• Keep the bioload (number of fish and other live inhabitants) low.

• Select a filter model with a rigid, sturdy undergravel plate.

• Use enough filter plates to cover the entire bottom of the tank.

• Cover the filter plate with 1-2 inches of appropriate gravel.

How to Maintain an Undergravel Filter

To maintain your undergravel filter, you must keep the substrate clean with regular gravel vacuuming. This is essential because the filter is designed to pull debris down into the gravel bed.

If you don't clean your substrate regularly, waste will be pulled under the filter plate, creating a haven for muck to settle and degrade water quality.

One effective cleaning method is to attach the inflow tube of a canister filter to the UGF's uplift tube. Place the outflow tube into a bucket and turn on the canister filter. This will suck accumulated debris from under the plate, cleaning the filter and performing a water change simultaneously.

Always ensure that your undergravel filter fits snugly in your aquarium. If there are gaps on the sides, debris will get trapped in these spots and become difficult to remove.

Types of Aquariums Best Suited for Undergravel Filters

While not ideal for every setup, undergravel filters excel in specific applications where their benefits can be fully maximized.

• Hospital/Quarantine Tanks
  These tanks are used temporarily and often have limited decor, making them an ideal choice for a UGF with a thin layer of gravel. They provide adequate biological filtration for a few fish. Avoid carbon inserts, as they will remove medications from the water.

• Breeding Tanks
  The low, gentle flow of an undergravel filter is beneficial for many breeding setups. For egg-scattering species, using coarse gravel or marbles over the filter plate can protect the eggs from being eaten while maintaining water quality. Sponge cartridges can also prevent fry from being sucked into the uplift tubes.

• Small Tanks
  Small aquariums, such as betta tanks, work very well with undergravel filters. They can be paired with an air pump and air stone to provide the low water flow that slow-moving fish like bettas prefer. It is also much easier to perform thorough gravel vacuuming in a smaller tank.

Frequently Asked Questions

Are Undergravel Filters Good For Shrimp?

They can be, but with caution. While the biological filtration is beneficial, small shrimp can get trapped under the plates. Meticulous substrate cleaning is essential to prevent dead zones that are harmful to them.

Can Undergravel Filters Be Used With Powerheads?

Yes. Using a powerhead instead of an air pump significantly increases water flow and filtration efficiency, enhancing the overall performance of the undergravel filter.

What Is The Best Substrate For An Undergravel Filter?

A two-layer approach is optimal. Use a layer of coarser gravel directly on the filter plates for good circulation, topped with a layer of finer gravel to trap detritus and host beneficial bacteria.

A two layer substrate approach provides optimal filtration results.

Do Undergravel Filters Remove Ammonia?

Yes, they are highly effective at removing ammonia. The gravel substrate provides a large surface area for beneficial bacteria to convert toxic ammonia into less harmful nitrate through biological filtration.

Are Undergravel Filters Outdated In Modern Aquascaping?

While a classic method, they are not entirely outdated in 2025. Their biological filtration is effective, but aquascapers should consider their impact on rooted plants, which can be managed with techniques like "blind potting."

After reviewing the undergravel filter pros and cons, it's clear they are a specialized tool. They excel in simplicity, cost-effectiveness, and biological filtration for fish-only tanks but require diligent maintenance to prevent issues. For complex planted or reef aquariums, other filtration methods may be more suitable. If you need help choosing the right system for your tank, the experts at Betta USA Aquatic are here to help.

Read more:

• The Beauty of Betta Fish: Why They’re Called the “Jewel of the Aquarium”

• How to Clean Sponge Filter: Steps for Clear Water

• Shrimp: The Clean-Up Crew of Aquariums