Regardless of the size of your tank and whether you have a freshwater or marine setup, an efficient biological filter system is essential to provide a healthy environment for your fishes and other livestock.
Why? Well, every living creature generates waste products, and that waste, together with uneaten food, dead plant matter, etc., triggers the Nitrogen Cycle to begin in your aquarium. Without a biological filtration system, the water in your tank quickly becomes overloaded with ammonia, nitrite, and nitrates, all of which will harm your fish, and may even kill them.
Of the various types of filter systems that are available today, the undergravel filtration system is regarded as somewhat old-fashioned and is often passed over in favor of more complicated technical systems. However, the UGF is currently enjoying a resurgence in popularity.
In this guide, we explain what’s so good about UGFs, how they work, and how to maintain one. And just for you, we’ve also tracked down and reviewed five of the very best undergravel filter systems on the market.
Quick Comparisons of the 5 Best Undergravel Filters
|Imagitarium Undergravel Filter||View Product|
|Lee's Original Undergravel Filter||View Product|
|Lee's Premium Undergravel Filter||View Product|
|Aquarium Equip Undergravel Filter||View Product|
|Penn Plax Undergravel Filter||View Product|
What Is An Undergravel Filter?
When I first got into keeping fish some 30 years ago, undergravel filters (UGFs) were the go-to filtration system for most aquarists, and they are still pretty much the most basic form of filter you can find.
A UGF is basically a rectangular plastic grate or plate that you place on the bottom of your aquarium, underneath the substrate. The mechanical element of the filter system pulls water down through the substrate and over the plate, providing a massive surface area on which waste-consuming bacteria can form vast colonies. That’s what makes the UGF the best choice if you’re looking for a highly efficient biological filter for your tank.
Basic UGFs that are powered by a simple air pump are best-suited for use in tanks of less than 55 gallons, although you can use them in larger aquariums with the addition of a powerhead that provides extra pulling power.
How An Undergravel Filter Works
An undergravel filter consists of one or several flat or slightly graded, plastic plates that are pierced with small holes or slits. You put the filter plate on the tank bottom and cover it with your chosen substrate. Gravel or coarse sand is the best substrate to use, as media that is too fine tends to block the holes and impede the water circulation across the plate and through the substrate.
Two vertical “uplift” tubes are fitted into circular slots in both back corners of the filter plate with the top of the tubes underneath the water surface. Inside each of the uplift tubes are air stones that are attached to the lengths of the airline. A pump or powerhead at the top of each tube draws water up through the tube along with bubbles that are generated by the air stones.
That suction action, in turn, pulls water down through the substrate and into the filter plate, feeding the beneficial bacteria colonies with dissolved oxygen as it does so. The result is a thriving mass of bacteria, an efficient biological filter system, and a healthy, well-oxygenated environment for your fish.
Solid waste particles floating in the water are dragged down into the gravel, eventually impeding the water flow rate and depriving the bacteria on the filter plate of oxygen. That means that the biological filtration element of the system gradually becomes less efficient, and the water quality deteriorates. For that reason, many hobbyists choose to use a secondary filter system to remove solid waste before it has a chance to clog the gravel.
Most UGFs do not contain a chemical filtration element. However, some come with an activated carbon cartridge or pouch that sits at the top of each of the uplift tubes. The problem with that arrangement is that the cartridges typically don’t remain effective for more than a few days before they require replacement. Also, placing a cartridge over the top of the uplift tubes tends to restrict the flow of water through the filter system.
Benefits Of Undergravel Fish Tank Filters
Undergravel filters have lots of benefits.
Reliable And Easy To Use
Compared with some of the more complicated and technical filters that you can buy, the UGF is remarkably simple, and with that simplicity comes reliability. That’s quite simply because there’s very little that can actually go wrong in a system that has no moving parts.
The mechanical element of the UGF is an air pump or a powerhead, and if you choose a reliable one, the system should give you no problems.
Aside from the gentle hum of the pump and the relaxing sound of bubbling water, a UGF system is completely silent, which is great news if you want to keep your aquarium in a workroom or bedroom.
Minimal Maintenance Requirements
An undergravel filter doesn’t have messy sponge filters or cartridges to wash or replace. You simply need to vacuum the gravel every week as part of a routine partial water change, just as you would with any kind of filter system.
Superior Biological Filtration
Compared with other filter systems that offer only a few square inches of media on which bacteria can grow, a UGF utilizes the entire floor of your aquarium! So, provided that you keep the gravel clean and remember to vacuum away solid waste particles, your water should remain super-clean, clear, and healthy for your fish.
Unless you have an external canister filter or sump that’s hidden underneath your aquarium in the cabinet, you’ll have a bulky, ugly box filter on view in your tank. That’s not the case with a UGF.
The filter UGF system’s filter plates are hidden beneath the substrate, and all you can see in your tank are two transparent plastic columns full of bubbles. If you don’t want to see the uplift tubes, simply place tall plants in a position to obscure them.
Cheap To Buy And Run
Undergravel systems are cheap to purchase and run. Once you have bought the unit and pump or powerhead, that’s it! You don’t have to buy replacement filter media or expensive cartridges, and there’s nothing to go wrong with the unit unless the pump breaks.
Suitable For Poor Swimmers
If you keep fish that are poor swimmers, a standard filter system that cannot be adjusted may put out a flow that’s too strong. For example, betta fish and fancy goldfish do not generally enjoy a strong water current, so a gently bubbling UGF is ideal for these guys.
An undergravel filter system can be customized relatively simply. For example, if you want a stronger water flow through the gravel, you can choose a hefty powerhead, rather than a small pump.
Also, you can include a chemical filtration element by burying a porous filter bag filled with carbon or Zeolite under the gravel on top of the filter plate.
The biological filtration element can be supercharged by adding a layer of filter sponge or floss over the filter plates, beneath the substrate. That innovation can be used in marine, brackish, and freshwater tanks too.
How To Choose An Undergravel Filter
Before deciding on a UGF, there are a few things that you should consider:
Some undergravel filter systems are constructed from one single plate, whereas others have two plates or even more. Before you choose a UGF, measure the bottom of your aquarium, and check the dimensions on the product packaging to make sure that one you choose fits your tank or can be trimmed to size if necessary.
Unlike other filtration systems that have an integrated power source and pump, UGFs do not usually come with one included in the kit. So, you need to buy an air pump or powerhead. Larger aquariums over 55 gallons generally need a powerhead to generate strong enough suction for the UGF to work efficiently.
As a general rule of thumb, when you place your filter plate under three inches of gravel, the flow rate that you need from your pump is at least 60 GPH (gallons per hour).
UGFs are perhaps not the best choice for you if you keep very dirty fish species, such as goldfish. That said, I did successfully use a UGF with a 30-gallon setup that housed fancy goldfish, although I was very diligent in vacuuming the gravel each week, and the pump I used was a large one.
As discussed above, if you want to run a system that includes chemical filtration, you’ll need to add activated carbon filters to the UGF system.
Undergravel Filter Maintenance
The main drawback to using a UGF system is that it causes particles of solid waste to gather between the pieces of gravel that make up the substrate. Over time, that blockage disrupts the passage of water across the bacteria in the substrate and down through the filter plates, depriving the bacteria of oxygen and creating pockets of foul water.
Now, that’s not a problem provided that you are diligent in carrying out partial water changes every week, as well as agitating and vacuuming the substrate to remove general detritus and fish waste.
It’s also recommended that you take down the tank completely every 18 months or so to remove and rinse the filter plates, clean the tank bottom, and swill the gravel through in aquarium water to remove accumulated muck.
Sometimes, algae grow inside and over the outside of the uplift columns. You’ll need to clean the tubes as and when required, using a soft-bristled bottle brush.
Best Undergravel Filter Reviews
In this section of our guide, we’ve picked out five of the best UGFs that are currently on the market and reviewed them for you.
1. Imagitarium Undergravel Filter
- 28.5 x 11.5 inches
You can use the Imagitarium UGF in both marine and freshwater tanks. The filter plates are suitable for use in a regular size tank of 10” x 19” and are designed to be used with a gravel substrate.
The system comes with two slatted filter plates that you place underneath the gravel on the bottom of your tank. Two uplift tubes connect to the filter plates and are supplied with air stones and extensions. You also get two activated carbon filter cartridges.
Like most UGF systems, you need to buy an air pump or powerhead separately.
2. Lee’s Original Undergravel Filter
- 7.5 x 15 inches
Lee’s Original UGF is available in a range of handy sizes, so you’re sure to find one to fit your tank.
The system contains just one filter plate that’s made from strong, durable plastic. A flow-through setup is included that accommodates a powerhead if you want to use one with your system. The super-strong material means that the filter plate won’t degrade if you use it in a marine or brackish tank.
Two adjustable uplift columns are included to provide excellent water circulation through the substrate and the filter plate. Also, you get airline tubing and air stones included with the kit. However, as with most UGFs, you need to buy a pump or powerhead separately.
3. Lee’s Premium Undergravel Filter
- 10 x 20 inches
Lee’s Premium UGF is a simple one-piece unit, so you need to choose the right size for your tank or buy multiple units if you have a large aquarium. However, the larger plate size means that you don’t need to buy as many filter plates for a big tank as you do with other brands, making this an economical choice.
The materials that are used to make the filter plates are tough and robust, so you can use the system with marine and brackish tanks, as well as freshwater setups. Unlike many canister or box filters, the large undergravel plates cycle the aquarium relatively quickly, so you can be up and running sooner.
4. Aquarium Equip ISTA Undergravel Filter
- 5.9 x 11.8 inches
This system can be used with a standard air pump, HOB filter system, or powerhead to provide increased circulation and include chemical filtration if you want that for your setup. The uplift tubes can be trimmed to provide the perfect fit for your aquarium, and the cleverly designed tube heads reduce noise.
You can use this undergravel unit in freshwater, marine, and brackish tanks, and the filter plate can be cut to fit as required. The air stones provided are of pretty decent quality, so they won’t degrade or collapse.
5. Penn Plax Premium Undergravel Filter
- 50 to 55-gallon
- 11.5 x 11.5 inches
The Penn Plax UGF is one of the most popular undergravel systems that you can buy.
The unit is suitable for use with 40 and 50-gallon aquariums and is made up of four individual filter plates that clip together. You also get two extra-large uplift columns, generous lengths of the airline, and air stones. As a bonus, you’re provided with two carbon filter cartridges that keep the water crystal clear and free from the unpleasant, sulfurous smell that sometimes afflicts UGFs.
This UGF can be used in marine and freshwater aquariums, the materials used are strong and built for longevity, and the system is easy to set up and maintain. Note that the filter plate connectors are quite fragile and can break easily, so take care when handling them and setting up the unit.
I hope you enjoyed this guide to choosing the best undergravel filter system for your aquarium.
Our favorite product is the Penn Plax Premium UGF kit. This is the most popular undergravel filter on the market at the time of writing and is perfect for those with larger tanks of 40 to 50 gallons. The filter plates come in four separate units that you clip together for the perfect fit. The uplift columns are extra-large for increased water flow, and the system also comes with two activated carbon filter cartridges.
If you have a smaller tank, you should choose Lee’s Original Undergravel Filter that comes in a wide range of sizes, is very well-made, and is suitable for use in both freshwater and marine tanks.