Filtration systems that incorporate a sump offer as many advantages to a free-standing tank as they do to built-ins. The only real difference is in the size of the sump, which must fit beneath the aquarium, hidden by the support cabinet. See the section on filtration for built-in tanks, above, for more information on sumps.

If you don't want to bear the extra expense of a drilled tank and sump filter system, plenty of other options exist.

Canister Filters

In this design, the filter media are contained within a sealed canister through which water is pumped. The inlet and outlet pipes are connected to the canister by hoses. Since the only equipment in the tank are the two pipes, they can easily be hidden with plants or decorations. The canister sits underneath the cabinet.

Hang-On Filters

The hang-on design is an old and reliable one, especially for smaller tanks. Water siphons out of the tank into a box that hangs from the top rim. After passing through filter media, it is pumped back into the tank over a spillway. Various brands add other features to this basic design.

Filter Media

Either type of external filter can contain two basic types of media: particle and chemical. The most commonly used chemical medium is activated carbon. It excels at removing compounds that tint the water yellow, and even traps some large molecules, too. Carbon has a short life span, however, and needs regular replacement. The pores in the carbon pieces become saturated with the substances extracted from the water. Despite this, the carbon does continue to function as a biological filter, because each piece becomes colonized with beneficial bacteria. Though not absolutely necessary, carbon filtration benefits almost any tank.

All sorts of plastic and fiber products are sold as particle filter media in aquarium shops. Their function is to trap suspended debris, giving the aquarium a tidier appearance. Like carbon, a particle medium will become colonized by beneficial bacteria, and thus does double duty. In fact, this may be the main value, especially in an aquarium that lacks living plants to help with fish waste removal.

I have found that external filter media supplied as a cartridge ready to slip neatly into the canister or a hang-on box are the least trouble to work with. Periodically, any filter medium will require flushing out and/or replenishment. Many aquarists find it most convenient to do this when performing a partial water change. If you are merely rinsing the particle medium, do so in the bucket of old aquarium water. That will help to maintain bacterial activity. Because changing the carbon or rinsing the particle medium with tap water drastically

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reduces the population of beneficial bacteria, I recommend only replacing a portion with new medium at each maintenance time. If you are using cartridges, you can leave the old cartridge sitting in the filter box for a week or so after you install a new cartridge, in order to keep those bacteria on the job.

Undergravel Filters

In this type, the gravel on the bottom of the tank is the filter. A perforated plastic plate sits on the bottom of the tank, with standpipes that reach the surface. Water is pumped up through the standpipes by small electric pumps called powerheads. This causes more water to flow downward through the gravel bed. Each piece of gravel becomes coated with beneficial bacteria.

The undergravel filter offers the least expensive option for efficient biological filtration but has some drawbacks. First, some plants do not like water movement around their roots, which occurs if they are growing above the filter plate. This won't matter, of course, if you are using plastic. Second, the gravel bed also acts as a particle filter and will become hopelessly clogged with debris if it is not periodically vacuumed with a specially designed aquarium siphon. Despite these negatives, I have enjoyed many a tank outfitted with only an undergravel filter. Combining an undergravel filter with an external filter also works well, giving you an extra measure of pollution control.

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