A substantial layer of gravel can act as a mechanical and biological filter media in aquarium set-ups called undergravel filters.
At its most basic set-up an undergravel aquarium filter is simply a rectangular grating or plate that sits under a gravel substrate and pulls water down through this substrate, thereby performing mechanical filtration and offering a vastly larger site for beneficial bacterial colonies to thrive.
In this article, I’ll discuss the benefits of under-gravel filters, their mechanism of operation, what you should look for in a filter that sits under your substrate, and I’ll review the best undergravel filter options on the market.
Benefits of Under Gravel Fish Tank Filter
Undergavel fish tank filters come with a series of advantages, especially if used in smaller or mid-size aquariums.
I recommend them for aquariums under 55 gallons, but experienced aquarists have been using them successfully even in larger tank set-ups.
Therefore, if you’re considering an undergravel filter for your tank, here are the benefits to look forward to:
Generally speaking, undergravel filters are an affordable filter option compared to other aquarium filter types on the market.
Because the under-gravel filter sits under the substrate (except for additional equipment such as air pump, air tubes, or power head, if used) it’s a space-saving aquarium filtration solution.
– Easy to set up
These filters are quite easy to set up when putting together your aquarium. Simply place the filter grid on the bottom of the tank, put on the substrate, and you’re basically done!
The only noise that you will hear with an undergravel filter is the quiet vibration of the air pump. If other aquarium filters seem too noisy to you, these UG filters may be a good alternative to consider.
How Undergravel Filter Works?
The mechanism of operation of these filters is simple — water is forced down the gravel, which traps debris and other particles, then it’s sucked through the tiny holes of the filter tray and returned into the tank via the uplift tube.
These filters can even be used in outside ponds, however, a power head is required to create an efficient current to force water down the gravel.
In these types of set-ups, the gravel is the filter media base that offers mechanical and biological filtration. The large surface area of the gravel is beneficial in being a site for healthy bacterial colonies.
Of course, keeping the gravel clean by siphoning it regularly is crucial in the maintenance of these filters, so make sure you know the steps involved in properly maintaining UGFs.
How to Choose an Undergravel Filter?
It’s essential to choose a good undergravel filter for your aquarium, and to do so, you must consider the following aspects:
– Aquarium Size
Regardless of whether they’re undergravel filters or not, the filter you choose for your aquarium should take the size of your aquarium into consideration. Check to see if the filter you’re choosing is rated for your tank size.
– Extra Components
As I mentioned, some UGF use air pumps others should be used with powerheads. The bigger your aquarium, the more reason to use a powerhead instead of a simple air pump, which may not be enough to create a strong enough current in your tank.
– Type of Filtration
If chemical filtration is not needed in your aquarium or you’re using the undergravel filter only as a secondary filter to a primary chemical filter, then there’s nothing further you need to concern yourself with.
However, if you need chemical filtration too, you shouldn’t rely solely on an undergravel filter that performs only mechanical and biological filtration.
Best Undergravel Filter Reviewed
Finding a good undergravel filter can be daunting, so here’s my take on the 5 best undergravel filters:
1. Lee’s 40/55 Premium Undergravel Filter
This UGF has a multi-level plate design made of resistant plastic that isn’t prone to splitting or cracking, therefore it will easily support the weight of your substrate plus the water column.
The product ships with two bottom plates and four riser tubes, and you can use the filter with a powerhead, canister filter inlet, or an aerator, or even all at once.
The plates are designed to fit a 48 inch long aquarium and it can be used in tanks of up to 55-60 gallons.
The Lee’s undergravel filter can be used both in freshwater and saltwater aquariums without issues.
Beyond offering mechanical and biological filtration, this UGF also comes with four activated carbon cartridges for those who can’t do without chemical filtration.
- Comes with multiple tube ports;
- Includes activated carbon cartridges;
- Made of durable, cracking resistant plastic.
- Filter tubes are a bit small.
2. Penn Plax Undergravel Filter
Suitable for 40 to 50-gallon aquariums, the Penn Plax undergravel filter comes with four filter plates and 1 inch adjustable rise tubes.
For each section, the manufacturer provides clips for holding them in place. The filter is safe to use both in freshwater and saltwater applications.
Like any UG filter, this Penn Plax is also designed to stay out of view, allowing your tank to be free of unnecessary equipment.
Because the lack of chemical filtration is often an issue with UG filters, the Penn Plax filter also includes two carbon block cartridges to remove odors, pollutants and to deal with discoloration issues.
Its simple installation and the added benefit of chemical filtration makes this UG one of the best undergravel filter options on the market.
- Chemical filtration included;
- Suitable for 40-50 gallon tanks;
- Easy to set up;
- Tube pieces are thin and fragile.
3. Aquarium Equip Undergravel Filtration Bottom Circular Bar
If you’re looking for a truly discreet undergravel filtration solution, Aquarium Equip’s circular filtration bars are something worth considering. It’s made to be used in aquariums up to 55 gallons, freshwater or saltwater.
Although a bit more difficult to install, it’s great for anyone looking to set up a custom-made aquarium.
You can use it even with a canister filter by connecting it to the inlet of the canister filter or you can use it with a powerhead.
Unfortunately, it won’t attach to standard air pump tubing, so get ready for some workarounds if you’re adamant on using this filter with an air pump.
- Very discreet UG filter;
- Great for custom applications;
- Suitable for both freshwater and saltwater tanks;
- Can be attached to a canister filter inlet;
- Works with tanks up to 55 gallons
- Difficult to install;
- Not readily compatible with air pumps (use powerhead instead).
4. XMHF Aquarium Fish Tank Undergravel Plastic Filter
This filter includes 28 pieces of UGF boards, uprise tube, air hose line and air stone for a custom-made assembly based on your tank size.
The XMHF filter can be used with an air pump, powerhead or attached to the inlet of a canister filter. It’s easy to assemble and fit in your aquarium.
Unlike the other filters I reviewed above, this does not come with a carbon filter, therefore, if your tank required chemical filtration as well, I wouldn’t recommend this filter as your primary filtration unit.
- Easy to assemble, fits canister filter inlets;
- Comes with air stones included;
- No chemical filtration.
5. Aquarium Equip ISTA Undergravel Filter
This is a basic UGF set-up from Aquarium Equip, which is very affordable, and it can be used with filter pump, air pump, submersible powerhead and hang-on-back style filters for better water circulation in the tank.
It’s suitable for smaller aquariums, its size parameters are 11.8-inch by 5.9-inch, so it will fit nicely in a 5-5.5-gallon tank.
The rise tube provided with the filter has an adjustable height and each plate can be connected with a tube. The product supports the installation of air stones as well.
Because it doesn’t include any other filter media, the gravel bed is the only filtration that will be used by this filter.
Activated carbon cartridges are not provided with the product, therefore, it’s not suitable to be used as a primary filter in applications where chemical filtration is also desirable.
- Affordable and easy to use;
- Works with air stones, air pump, powerhead, canister filter.
- No chemical filtration.
What Substrate is Suitable for Undergravel Filtration?
The name of these filters is a dead giveaway as to the media base that’s most suitable for these filters — gravel. So, will any aquarium gravel do? Not exactly.
First, the gravel substrate must be at least 2 inches thick for it to have a beneficial filtering effects, if it’s too thick or too thin the filtering process won’t have the desired effects.
Next, the gravel should be coarse, but not too coarse, otherwise it won’t trap particulates. If the gravel is too fine, it can clog the filter tray causing malfunction and undermining the filtration.
Therefore, sand is out of the question, which leaves us with a gravel that’s not too coarse, not too fine, but somewhere in between.
Another thing to consider is your fish and whether the gravel you choose is suitable for them or not. Some fish may get hurt by gravel that’s too coarse, other fish may accidentally ingest the gravel if it’s too fine.
Therefore, you must find a gravel that will tick all these boxes for you to place on top of your filter.
Undergravel Filter Maintenance
One of the major complaints about UGFs is their long-term maintenance requirements. A maintenance routine is important with these filters, because detritus and organic matter can build up in the substrate.
Therefore, regularly siphoning the substrate is a good way to stay on top of the issues that could arise if too much junk builds up in the substrate.
It’s also crucial to use a good air pump/powerhead to make sure air flow gets everywhere under the gravel as dead spots can benefit the growth of bad bacteria.
Besides siphoning the substrate on a regular basis, you could also use a pre-filter (HOB or canister) to remove free-floating particles from your tank and reduce the amount of gunk that gets into the substrate.
Overfeeding your fish can also cause materials to accumulate and work their way into the filter bed, so avoid overfeeding and avoid overstocking your tank, which also produces more waste.
Therefore, there are four things you can do to avoid problems in your aquarium and in the proper functioning of your undergravel filter:
- Avoid overstocking the aquarium;
- Avoid overfeeding your fish;
- Siphon the gravel and on alternate occasions siphon the space beneath the filter grids;
- Use a pre-filter to capture as much of the free-floating particles and possible.
Taken together, these filter maintenance tasks, if performed routinely, will make sure you won’t have to tear down your tank and set it up anew because of too much dirt that has accumulated.
Can You Use a Canister Filter with Undergravel Filter?
Yes, you can use a canister filter with an undergravel filter by connecting it to the canister filter inlet. The UGFs I reviewed above are designed to work with canister filters too.
In fact, having a multi-filter set-up is even better for your aquarium because some undergravel filters are not equipped with chemical filter media.
Undergravel filters may not be the most popular choice on the market, but they’re certainly an option you shouldn’t dismiss if you’re looking to set up custom-made aquariums and you don’t want to keep too much equipment in the aquarium.
UGFs are straightforward in the way they work, they’re easy to set up and one of the more silent filtration methods out there, but they do require enhanced attention to maintenance.
If you want to set up an aquarium with a UG filter system, check out my recommendations for best undergravel filter and pick the filter that meets the requirements of your aquarium.
If you’re going to use these systems as your primary filter, make sure to pick one that also offers chemical filtration and remember to devise a fail-safe aquarium maintenance routine.