/The Conscientious Aquarist

Freshwater Undergravel Filtration, Tried and True

By Bob Fenner

Undergravel filters, uninstalled

Of all the gear for aquarium keeping, arguably the lowly undergravel filter is most maligned and underappreciated. Though many pet-fish authors scoff at this "old-timey" technology, indeed making reference to scooting wastes under the rug in comparison to its action, undergravel filtration is a stalwart technology for many if not most freshwater aquariums.

What U/G Filters Are:

Undergravel filters comprise some type of plates, screens or notched support that gravel is poured on top and/or around of. A few standard approaches are used to either draw water through these grids ("regular" or gravity flow) or the opposite way (reverse flow), blowing water through the plates, up through the substrate. The filter panels can be one piece to a few, with airlifts or connections to powerheads or "blind" with pumping being remoted and holes drilled into the tank to service them.

How Undergravel Filters Work:

U/G filters employ your gravel as filter media, along with food, waste and ash particles and microbes to trap suspended materials and through biological and chemical methods, convert these into gasses and allow their easy removal by gravel vacuuming (siphoning). By appropriate (not too fast or slow) drawing of water through your aquarium gravel, wastes, uneaten food and such are caught between the gravel particles interstitial spaces. Ready flow of water provides oxygen to beneficial microbes (bacteria and more) that break-down much of this material.

Obviously, the size, shape and depth of substrate you use is important to undergravel filtration functionality. As a general rule, the larger, more spherical the gravel is, the greater depth you will need to employ. In practical terms, most systems are "about right" with an inch and a half to two of gravel over their U/G plates. Should your system remain cloudy, it may be that you need more gravel or alternatively to slow down the rate of flow through your U/G plates.

Not A Panacea: U/G Filter/System Maintenance:

Chief valid complaints about undergravel filters focus on their capacity for accumulating wastes that are not easily removed, however all types of filtration have the potential for this sort of failure and therefore it is instructive to list the principal inputs for filter failure and their remediation and avoidance. Too much waste in the gravel: This is a function of either too much feeding, the feeding of inappropriate foods, too little regular maintenance and/or a lack of co-filtration.

U/G Plus Other Filtration:

In almost all cases, utilizing other types of filtration with undergravel is warranted. Outside power filters (hang-on, canister, sump-types, cartridges...), inside power filters, and more are useful to have/employ not only for their added "polishing" effects, but for their enhanced circulation, aeration and redundancy should your water moving mechanism/s fail on your undergravel. I would always run another type of filtration in concert with undergravel. Think of the times when you may want to add chemical filtrants (in a porous bag likely). Where are you going to place this filter media? In your ancillary filtration.

Regular Maintenance:

Ongoing water changes, gravel vacuuming to free-up spaces in your substrate, prevent anaerobiosis there is mandatory. Depending on your system size and type, some 10-20 % of water should be changed out every week or two, while either gently swirling your gravel by hand or with a tool, or better, using a "gravel vacuum", a siphon with an expanded end, to stir up the gravel, remove the matter in-between while leaving the gravel behind. There is a bit of methodology to this, and some practice involved to become facile at the operation. Do be careful not to be too fastidious in your gravel vacuuming, as the removal of too much of the useful microbial populating in captive systems can lead to bottle-necking your biological filtration. A good plan is to gravel vacuum only one half of the system at each cleaning interval, leaving the other side to "take up the slack". Similarly, it may well be prudent to change, clean other filter media in accessory filters on alternate dates.

A note re the need to keep your plates covered. There are many motile aquatic animals (cichlids, crustaceans...) that like to dig, move their gravel about. For these systems, having more than one filter plate and other filtration becomes even more important, as the uncovered areas become the paths of least resistance and consequently diminish the effectiveness of the plate/area that is uncovered.

About U/G Filters and Live Plants:

Rooted aquarium plants and undergravel filter plates are often cited as mutually incompatible, with various notions offered to explain why plants don't do well with water circulating too much about their root structures. You are encouraged to either "blind pot" your rooted plants or place a barrier such as a flat piece of plastic over sections of your plate/s between the gravel and the plate itself, in areas where you'll be planting, and to avoid gravel vacuuming there.

In Conclusion:

In the mid 1990's when penning the Conscientious Marine Aquarist, owner/editor James Lawrence of Microcosm was apparently dismayed at my inclusion of undergravel filtration as a means of marine aquarium filtration; "That's so old"... "there are much better methods nowadays". I told James what I'm writing here for you, that U/G is a tried and true method of readily filtering biological aquatic systems. Yes, not the only or even the best, but for the money involved, ease of use and reliability undergravel filtration is a sure provider of viable water quality. Coupled with other powered types of filtration, in and/or out of the system, undergravel filtration can provide clean, clear water for almost all types of freshwater set-ups.

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