by Bennett Kalafut

Created on: May 30, 2008

Dissolved oxygen very often limits the rate at which Nitrosomonas and Nitrobacter bacteria can oxidize ammonia to nitrite and nitrite to nitrate in the aquarium. A wet-dry filter, in which water is splashed or trickled over a porous medium, wetting it without immersing it, provides a large water-to-air interface, substantially increasing oxygen available for nitrogen waste metabolism. This benefits the fishkeeper by damping out "spikes" in ammonia or nitrate concentrations caused by the death of a fish or invertebrate and increasing the overall carrying capacity of the aquarium.

Marineland/Penguin sells "Bio-Wheel" attachments allowing aquarists to add a wet/dry area to any canister filter setup. Better still, one can build a trickle filter from inexpensive components. A simple design used in small aquaria consists of an acrylic "refuge" box (the sort the pet store puts fish in before bagging) or similar container, hung inside the aquarium, with holes drilled in the bottom and filled with aquarium gravel or commercial ceramic biological filter bed medium. The top of this box should be a couple of inches above the water level. An air-pump powered sponge filter with a ninety-degree elbow is affixed to the side of this; when the pump is turned on, the sponge filter will splash water into the box, which will then trickle back to the level inside the tank. This is an adequate system for a small (five to ten gallon) aquarium and a fine method of filtration for a fry rearing tank-one can even incorporate a layer of activated carbon in the bed-but does not scale very well. An external trickle filter, similar in principle, provides the same benefit and more.

The external trickle filter is mounted in a sump, usually placed under the show aquarium in a cabinet, but easily placed anywhere else so long as the return pump can, accounting for loss of head, cycle the water through the filter at the rate desired by the user. Cheap ten-gallon aquaria are commonly used as sumps, but any inert watertight container will do, including plastic Rubbermaid tubs or even a large fish shipping bag mounted in a sturdy cardboard box. In addition to being a good place to mount a trickle filter, sumps make convenient refuges for injured fish and locations to mount other filtration devices such as Nitrex boxes and coil denitrators. It is not unheard of, either, to grow plants in the sump, either for filtration purposes-fast-growing Najas grass or Java Moss is an effective means to nitrate removal-or

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