Aquatics Unlimited 3550 S. 108th Street Greenfield, WI 53228-1257 Phone [414] 543-2552 Fax: [414] 543-4929

The Undergravel Filtration Controversy

The theory of undergravel (UG) filtration. It is generally acknowledged that there are three forms of filtration: biological, in which ammonia and nitrite of fish waste is broken down by bacteria, chemical, in which carbon or resins are used to adsorb dissolved waste, and mechanical, in which solid waste is strained from the water by floss or filter cartridges. Of these, a UG filter effectively provides only biological filtration. The small amount of charcoal in a UG cartridge does very little chemical filtration before becoming saturated and whatever mechanical filtration occurs is actually a threat to the stability of the aquarium (more about this later). In theory, the UG filter circulates water through the gravel, where healthy bacteria grow and break down the fish waste. Promoters of UG filters often argue that you cannot culture these bacteria without an UG filter, that ammonia and nitrite levels will rise and fish will die.

Our experience has been just the opposite. As you look around at our 200 aquariums, loaded with 5 to 10 times the usual number of fish per gallon, you will find no UG filters in either freshwater or saltwater. And yet, we never find dangerous ammonia or nitrite levels in any of them. It seems that the bacterial culture grows perfectly well on the gravel, the glasses and any other surface in the aquarium that has water circulating around it. We perform hundreds of water quality tests for our customers each month, and the vast majority of ammonia and nitrite problems in established tanks involve UG filters. Declining pH and live plant problems are also much more common in UG filtered tanks. When the culture is dependent on the UG system, an otherwise minor mistake or mechanical failure can cause a die-off of the bacteria and fish loss. When we used UG filters on our saltwater aquariums a few years ago, we often lost an entire tank full of fish overnight when a power head would quit. The power head would stop due to gravel packing, salt spray penetrating the motor or an anemone blocking the intake, and the bacteria (and fish) would be dead before we noticed it. In more recent cases when a power filter would quit, there were no major fish losses unless aeration was also interrupted.

The risks of UG filters. If all else is perfect - if the fish are not overfed, if circulation is not hindered, if the tank is not medicated, and if the gravel is cleaned frequently with water changes - success can be achieved using UG filters. We stock UG filters and power heads mostly for those experienced hobbyists who have had previous success. Unfortunately, in most cases, something is less than perfect. Usually, after a few months of operation, the gravel begins to pack with so much silt and dirt that circulation stops and the bacteria suffocate. Since the filter still appears to be working, water quality deteriorates, usually unnoticed until it is too late. If the tank is medicated or overfed, an even more rapid die-off of the culture may occur. A speedy water change may avert disaster, but this is the sort of headache and frustration we are trying to spare our customers.

The best filtration. We have had excellent success with box filters for smaller aquariums and power filters or canister filters for larger aquariums. They usually cost less than an UG filter with a pump to provide similar output. In addition to providing circulation for our biological filtration, they also are far superior mechanical and chemical filters. And since solid waste is collected in filter fiber or a filter cartridge and is easily disposed of, it is not allowed to accumulate and rot in the aquarium. Tanks stay clearer and odor-free with less maintainence by the hobbyist. And, as we mentioned above, we have found much less risk of problems of ammonia or nitrite poisoning.

Dual filtration? A number of customers have asked if perhaps the best aquarium setup might employ both a power filter and an UG filter, and at first we thought this might be the case. For quite some time, all our larger saltwater aquariums utilized both a canister filter and an UG filter with power heads. Filtration was quite adequate, but keeping up with packed gravel beds and self-destructing power heads was nearly overwhelming. We finally removed the UG plates (never simply "turn off" an UG filter) and found that the tanks stayed even cleaner with just the canister. The UG filter actually was competing with the canister filter and pulling much of the solid waste into the gravel - out of the reach of the canister.

Undergravel sponge filters. Since we originally prepared this article, we have experimented with using and selling Dirt Magnet TM sponge filters in many of our setups. We have found these $7 items to perform all the beneficial functions of $20-$30 undergravel filters, without pulling most of the solid waste into the gravel. In addition to provide a place for biological filtration to take place, they also help eliminate the "hazy" or "cloudy" water conditions that arise from overfeeding or overcrowding. Similarly, many of the new "wet/dry" add-ons to power filters can provide superior biological filtration with much less risk.

A final word. We have recently been pleased to see a number of articles stating the drawbacks of undergravel filters and citing the success of fishkeepers not using them. Our experience has shown that not only can an aquarium be successful without an undergravel filter, but that the risks of undergravel filters usually outweigh any benefits. We have held this opinion since we opened in 1969, and we feel that a portion of our success as a business stems from the continued success of our customers who use more efficient filtration.