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 for later aquascaping use.
The sump must have a return pump, chosen carefully from manufacturer specifications. Many aquarists merely keep this running all the time, but controlling it with a commercially available float switch is good practice; pumping the sump dry can ruin a pump and exacerbate the problem that caused the sump to run low in the first place. The return outlet can be placed over the water in the show tank, or slightly below it provided that the amount of water that siphons backwards into the sump during a power failure does not cause it to overflow. Remember: water damage is expensive, but five minutes of arithmetic is cheap.
A trickle filter, made from an inert plastic bucket or similar container, can be placed (adequately supported from the bottom or even hung by its handle from the top) in the sump, with most of its volume above the sump's set water level. Construct the trickle filter by cutting a large hole in the bottom of the bucket, gluing a rigid plastic grate over the hole (on the inside, so that it is supported by the uncut part of the bottom), and filling the bucket most of the way full with plastic "bio-ball" or ceramic media. Place a diffuser plate, cut to fit tightly in the bucket, over the bio-balls. The diffuser can be made of a piece of plastic egg-crate with holes drilled in it or even plastic peg-board material.
The filter's intake can be constructed in three ways. The least advisable is an active siphon, over and out of the top of the aquarium; this requires careful determination and balancing of the flow rate through the siphon and back up the return line; too fast a siphon rate can lead either to a wet mess on the floor or a water intake above the water level and filter that will not function until the siphon is restarted. More common and nearly foolproof are stand-pipe or overflow intakes. Both operate on the same principle; water above the intake level will spill into a pipe directed to the trickle filter. An overflow intake consists of a bulkhead fitting mounted in the side of the aquarium; a good glass shop should be able to drill the necessary hole. This permanently sets the aquarium's maximum water level; a standpipe is a better option. A rigid tube (made of, e.g. PVC) is fit to a bulkhead fitting at the bottom of the aquarium, with an intake strainer or sponge at its top. A hose attached to the bottom of the same bulkhead fitting directs water to the trickle filter. The water, spread out by the diffuser plate, trickles through the column of biomedia (this is the wet/dry part) and into the sump, where it is then returned to the aquarium by the pump. Placing a ball valve inline below the bulkhead fitting is a good idea as it allows the flow of water into the trickle filter to be quickly stopped for maintenance.
More elaborate trickle columns can be built. For example, one can modify the simple "bucket" setup so that the biomedia are held in a tray and the water must also subsequently pass through a canister of chemical media before entering the sump. Flexibility is the ultimate advantage of the trickle column/sump system; consider the purposes for which the filter is needed, draw up plans, and use your imagination.