The Undergravel Filter in Freshwater Fish Tanks

Aquarium filtration is a heated topic.While most experienced aquarists, including myself, recommend the use of canister filters in freshwater fish tanks, some “experts” continue to advocate the use of filters that are bulky, inefficient, and obsolete!

In particular, some people still use the undergravel filter (UGF), and many aquarium stores and pet shops continue to sell these obsolete filters. My sincere advice is to stay away from these ancient filter designs.

In The Kick-Ass Aquarium, I explain the drawbacks of UGFs in detail, and I point to a few great alternatives. If you read my book, you understand how UGFs work, so I won’t spend more time on the matter. Instead, I’ll talk about the flawed arguments supporting the use of undergravel filters.

One recent argument comes to mind. In his article, “In Defense of the Humble UGF,” Gio Maletti suggests that undergravel filters are well suited for growing live aquarium plants, and that they are efficient and barely visible.

Plant roots do best in fish tanks that have a sand or clay bottom (both materials look great, by the way), and they don’t tolerate a lot of water movement through the substrate, through the bottom material.

Well, the UGF can only be used with gravel because sand will clog this type of filter. Also, UGFs will produce a lot of water movement through the substrate!

Maletti’s solution to these two drawbacks seems simple. He suggests placing plants in pots that contain sand or clay in order to alleviate the two problems caused by undergravel filters (Maletti 95). So, now we need plant pots to make UGFs work? Apparently, the list of equipment needed for using undergravel filters is growing!

That brings us to the next issue: bulkiness. UGFs consist of a large plastic tray and two or more wide plastic tubes that protrude from the bottom of the fish tank. The tray will take up an inch of vertical space, thus decreasing the height of your aquarium by that amount! Also, the plastic tubes are far from inconspicuous! On top of this, you’ll need those bulky plant pots to make the whole setup work!

To Maletti’s credit, he correctly asserts that undergravel filters encourage biological filtration (95). But, there are much better ways of promoting this very important filtration method.

First, keep in mind that the beneficial bacteria that provide biological filtration naturally occur in the gravel or sand bed. In fact, sand has a much larger total surface area than gravel, so a sand bottom will house more beneficial bacteria than will a gravel bottom. The more poop-eating bacteria you have, the cleaner your water will be. Again, UGFs can only be used with gravel.

Also, you can increase water flow through the sand or gravel bottom, but to a degree that won’t harm plants, by taking a simple step that will actually make your aquarium look nicer!

By simply shaping the sand into a wavelike surface, as opposed to a completely flat surface, you will significantly increase water flow throughout the sand or gravel bed. And that’s all there is to it!

Finally, why rely on a bulky UGF to promote biological filtration when you can actually increase the beneficial bacteria population in your aquarium by attaching a canister filter to your fish tank?

With a UGF, you will only encourage the biological filtration that is already occurring in the substrate. As we discussed above, doing things this way is problematic.

By using a canister filter, on the other hand, you will increase the total amount of biological filtration! That’s because the beneficial bacteria will now have two places to colonize: the sand/gravel bed and the canister filter!

In short, UGFs are cheap (in terms of both price and quality), and any money you’d spend on them you would be wasting!


Maletti, Gio. “In Defense of the Humble UGF.” Tropical Fish Hobbyist, November 2006, 94-95.

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