Under Gravel Filter – What Is It & How Does It Work?

The under-gravel filter is a filtration system in which a grid is placed under the substrate gravel. The tank substrate often fills up with rotting plants, organic matter, and chemicals like ammonia and nitrites. 

By using a pump to sift waste from the gravel, your tank stays cleaner as the debris is vacuumed into the grid or tubes under the gravel. While a regular filter cleans the water, an under-gravel filter cleans all the sinking muck. Let’s learn more.

What Is an Under-Gravel Filter and How Does It Work?

The under-gravel filter has a rectangular grid (in most cases, made of plastic, but can also be made of conjoined pipes with holes) connected with a pipe that allows water circulation. The tank’s water pump connects to the tubes from the under-gravel filter, drawing gunk and water through a smaller filter at the top of the uplift tube before returning cleaned water to the tank.

The under-gravel filter fits at the bottom of the fish tank, right on the tank’s glass, with the gravel layer on top of the filter grid.

The under-gravel filter draws muck from the gravel, but it also uses the gravel as filtration media. Biological filtration happens because beneficial bacteria make colonies on gravel instead of just floating around in the water like malicious types. Since the pump draws water through the gravel, it also ensures the water is cleared of ammonia by the good bacteria.

The filtration cycle reaches completion by returning the water from the under-gravel filter into the aquarium.

The under-gravel filter has tiny holes through which water moves into the filter pipe, and it uses an air stone combined with a water pump to force the water up and return it to the fish tank.

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The Working Mechanism

The under-gravel filter works with air pumps. Air is lighter than water, so bubbles push the water up the filter pipe where it gets cleaned, then the water falls back into the aquarium. However, you can also use a water pump to draw the water into the filter pipe.

For the air filter to work correctly, the air tubing is threaded down the uplift pipe. The airstone at the bottom creates water flow when air bubbles up, carrying water with it.

Using an air pump is more effective on smaller tanks than large ones, where a water pump is better.

Use the under-gravel filter with a water pump (power head), which, when correctly placed in the upper end of the pipe of the under-gravel filter, creates a more considerable water flow for more effective filtration.

A water pump is also needed when an under-gravel filter is used in an outside pond.

Since a pond is very big compared to a home aquarium, a strong power head is used to create a flow of water that goes through the gravel. When you use an under-gravel filter for your koi pond, build an additional retaining wall underwater. Hence, the gravel stays in place to form a thick enough filtration material.

The setup is the same: first, you place the under-gravel filter grid, then you put the gravel on it.

Connect everything, turn on the water pump, and water is drawn through the gravel, through the small holes of the under-gravel filter grid, up the uplift tubes, and returned.

By cycling the water through gravel with colonies of beneficial bacteria, you have biological filtration, which helps rebalance the nitrogen and ammonia content of the water.

When Your Filtration Material Becomes Dirty

The gravel forms your filtration material, and, as you know, filter material requires regular cleaning to remove sludge and larger particles that may block the water flow.

The best way to clean your gravel is by using a gravel cleaner.

By siphoning the gravel, all the debris and organic waste build-up goes down the drain pipe into a bucket.

Once the gravel is clean, your filtration material is ready to be used again. Never scrub your gravel, as you don’t want to remove beneficial bacteria colonies. If you must rinse it, use water from your tank or pond.

When to Consider an Under-Gravel Filter

The under-gravel filter is less effective than other filtration systems, so don’t use it as the only filter in a large tank. It might be useful for smaller-sized fish tanks, but for bigger tanks, use it as an additional filtration unit, with a regular filter cleaning the bulk of your tank’s water.

Remember: the under-gravel filter does not provide a full filtration spectrum with proper mechanical and chemical filtration (which are essential parts of the filtration process). At most, the gravel will trap larger particles (a weak form of mechanical filtration).

The real value of an under-gravel filter is the colony of nitrifying bacteria you are cultivating in the substrate, which helps to remove ammonia as the water circulates over it. 

So, for the benefits of mechanical and chemical filtration in your tank, you need a hang-on filter, canister filter, or in-tank filter unit. If your under-gravel filter works well, the water remains clear, and the fish look healthy and happy.

However, suppose the water starts to turn green and cloudy, or the fish start to become diseased. In that case, you might consider additional filtration sources, especially chemical filtration, such as those offered by a hang-on-back or in-tank filter.

1. Penn-Plax Under-Gravel Aquarium Filter

Penn-Plax is one of the original filter manufacturers on the market, and their under-gravel filter grid is elegant and efficient. This particular under-gravel filter features an air pump design, which means that air is pumped to the bottom of the air lift tubes, creating circulation as the air bubbles through the tubes. 

Each tube has a filter-a-carb filter unit that filters the water, removing impurities, larger particles, and ammonia from the tank’s substrate for crystal-clear water.  

Technical Information

• Perfect for 40-55 gallon tanks
• Air filter design aerates tank water for healthy fish
• Four filter plates that connect according to tank dimensions
• Each filter plate size: 11.25” (W) x 11.25” (D) 
• Height-adjustable lift tubes allow for different tank depths
• Includes four filters
• Straight air tubes included (can order the air tube and air pump separately)

What We Like About It

With the basic air filtration design, your tank has a cleaner, more minimalist design that makes for great viewing pleasure. Your fish will benefit from having cleaner substrate gravel and clear water, plus the additional aeration aids in ensuring your fish are more energetic.

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2. Lee's 15/20 Original Under-Gravel Filter

Another quality manufacturer in the aquatic market is Lee’s, and this is one of their more basic under-gravel filter units. The grid is made of high-quality plastic to resist cracking or splitting. It’s perfect for small and nano aquariums. 

Use it with an air pump or add a power head for a stronger current and more filtration power. The unit includes a carbon cartridge filter and air diffuser to ensure optimal water filtration. Use this under-gravel filter with either a freshwater or saltwater tank, and as long as the substrate particles are large enough not to filter through the plastic grid, your tank will have clear water.  

Technical Information

• The size of each grid plate is 12 inches x 24 inches (two included in the box)
• Includes one uplift tube
• Includes filter and air diffuser  
• Best for 15-20 gallon aquariums
• Detailed assembly instructions on the back of the pack

What We Like About It

It’s ideal for beginner aquarists as the pack has all the instructions you need to complete the easy installation and assembly of the under-gravel filter. Spend your hard-earned cash wisely by purchasing this unit as it has been made from high-quality plastic and is sure to last.

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3. CORISRX BEST OF YOUR LIFESTYLE Under-Gravel Filter

The Corisrx under-gravel filter is similar to Lee’s 15/20 under-gravel filter. The difference is the plastic grid and the color. If you have white substrate gravel, you’d do well to opt for this unit which is made from white plastic, creating a luminous tint to your gravel. 

Connect the included uplift tube to an air pump or water pump to create a thorough filtration cycle. Pair the under-gravel filter with a hang-on-back pump for chemical and mechanical filtration and more pristine tank water. 

Installation is easy, and with the adjustable air lift tubes, your under-gravel filter will start cleaning your tank from the first day. 

Technical Information

• under-gravel circulation helps remove debris and decaying plant matter
• Excellent nitrification of the gravel layer thanks to the larger plate slots
• Set the uplift tubes to the height of your tank
• Size of plates: 11.8 inches by 5.9 inches (two plates)
• Install air stones in uplift tubes for additional water aeration

What We Like About It

For a tank that has a partial substrate, it’s best to use an under-gravel plate that matches the color of your substrate, and most plates are black, so this white plate set is ideal for white or light-colored substrate.

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4. Penn-Plax Small World Fishbowl Under-Gravel Filter Kit

Fitting a square grid into a round tank can be a challenge, but Penn-Plax jumps to the rescue. The circular grid is perfect for a round tank or fishbowl, and the green plastic design makes for an interesting overall finish. 

While the grid design fits into a bowl with a four-inch opening, modify the grid by breaking the outer rings away to fit a smaller bowl. Best of all, the set includes a small air pump, a disposable filter, air tubing, and an adjustable flow valve.   

Technical Information

• Adjustable size for different tank and fishbowl openings
• Unique coloring for a smaller-size tank
• The set includes all the necessary equipment, including the grid, air pump, filter, and more
• A new filter unit is available, or you can clean out the filter chamber
• Size of the grid: 7 inches by 5 inches by three inches

What We Like About It

The unique approach of the filter plate and unit makes this an ideal under-gravel filter for small, round, or cylindrical tanks. Adding the air pump and filter makes for a beginner-friendly under-gravel filter set.

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5. XMHF Aquarium Fish Tank Under-Gravel Plastic Filter Board Air Tube 28pcs Black

Create your own under-gravel filter layout with the 28-piece under-gravel set from XMHF. The grid features a dense structure for smaller particle substrates, while 28 pieces give you various layout options. 

The plates connect to create a larger surface, while the kit includes an air tube, uplift tube, and filter. With the unique interlocking design, you can place the uplift tube on any plate or add additional uplift tubes and air lines to create a stronger circulation system and filter.    

Technical Information

• Each plate size: 5.5″ x 2.8″ x 0.5“
• The grid of 28 plates is easy to assemble
• Includes grid, air stone, air hoses, and a strong filter tube with a filter
• Great price

What We Like About It

The grid may not be as versatile to connect as other grids, but the overall quality is great, and the grid pattern allows for smaller substrate material as biological filter material on the grid.

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6. Ipetboom Under-Gravel Filter Aquarium

Another excellent filter plate in white is the Ipetboom under-gravel filter. The kit has five filter plates, an uplift tube, and a straight air tube. The design of each plate has several openings to allow great circulation and better nitrification of the ammonia in your tank. 

The plate’s supporting “pegs” help keep the plate level, no matter the weight of the substrate on it, and you can easily purchase additional plates to connect and expand the plate system. 

Technical Information

• Square plate size: 5.82 inches x 5.82 inches 
• Uplift tube length: 9.84 inches 
• The plate has a unique criss-cross design for better filtration 
• Plates are reinforced with weight-bearing pegs
• Fairly priced

What We Like About It

We love the unique grid design of the plates, and each plate is sturdy and capable of supporting the tank substrate without sagging.

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7. Imagitarium Under-Gravel Filter

For a classic under-gravel designed filter, the Imagitarium set is perfect. The plate system comprises two plates, two uplift tubes, two filters, and air tubing. With two uplift tubes, there’s twice the filtration and circulation power, ensuring excellent tank hygiene. 

The filter plates are ideal for freshwater or saltwater tanks, and your fish will enjoy oxygenated water thanks to the two included air stones you can connect to your air pump. 

Best of all, the under-gravel filter’s tube filters are compatible with two brand-name replacement cartridges, making the unit’s maintenance super convenient. 

While this unit is compatible with a 10-gallon tank, there’s a 29-gallon tank plate system available too.  

Technical Information

• Plate size: 10.93 inches by 9.5 inches (two plates included)
• Each plate has fitment slots for the two uplift tubes 
• Slitted grid design accommodates most substrate material that won’t pass through
• Perfect for any aquatic environment
• Fairly priced

What We Like About It

The plates with two fitment slots for uplift tubes are really innovative, and the overall quality of the plates is impressive. All you need to add to the kit is an extra air tube to connect the tubes to the air pump, and you’re ready to filter.

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Our last option is a set of six plates (with four plate grids each) that includes one uplift tube and an air stone and air tube. The plates’ grid structure is secure, and the grid extends down the side of the plates for extra filtration.  

You can connect the air pump to the top of the air tube, or you can connect the water pump for a more powerful circulation effect. It’s best to use this under-gravel unit with a hang-on-the-back filter. 

Technical Information

• Plate size: 11 x 5.5 x 0.4 inches each (tray)
• Uplift tube length: 12.6 inches 
• Assemble the plates according to your tank dimensions
• Plates have a strong grid structure with middle support pegs

What We Like About It

The quality of the grid structure is quite sturdy, and your substrate gravel is unlikely to cause any sagging or blockage of the grid. The interlocking mechanism is strong enough to maintain the grid’s integrity but quickly loosened to rearrange according to your tank’s design.

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Finally…

An under-gravel filter system has many benefits, from cleaning the water and removing debris from the aquarium bottom to ensuring a subtle current or tank flow. It’s well worth it to get an under-gravel filter system for small tanks or fish bowls if you want to streamline your filtration system and have less piping and unsightly filters to contend with. For larger tanks, you’d do well to combine it with a hang-on-back filter. 

If you’ve decided on getting an under gravel filter, below are some of my recommendation: