Although many different types of aquarium filters exist, none is more satisfactory for the installation we are considering than the so-called wet/dry system popular with saltwater aquarists. I value this type of filter more for the basic plumbing scheme than for its superiority as a biological filter. In its simplest form, a second aquarium tank, known as a sump, sits underneath the main display tank. The display tank has a drain hole in the bottom, with a standpipe projecting above this drain. The height of the standpipe determines the water level within the tank. The standpipe is connected to the sump by pipes or hoses. An electric pump moves water from the sump via a pipe that reaches the top of the aquarium, where it discharges into the tank. When the pump is turned on, water from the sump causes the display tank to overflow the standpipe, completing the circuit. The total capacity of the system equals the tank capacity plus the capacity of the sump.

The standpipe inside the main tank is usually surrounded by an opaque plastic box with notches at the top over which water flows. The idea behind this design is to trap floating debris and surface film while protecting surface-dwelling fish. Some species, however, may be so inclined to dive over this waterfall that in order to exclude them you need a fine plastic mesh instead of narrow notches. Maintenance is required to keep the mesh free of obstructions such as dead leaves.

Efficient biological filtration occurs when a filter medium is placed in a box or on a tray between the tank drain and the sump, where it is showered constantly with water. Beneficial bacteria colonize this material and detoxify pollutants produced by the metabolism of the fish and plants in the display tank. When live plants grow in abundance as part of the display, much of this filtration capacity is redundant, and the filter medium can be dispensed with, although it does perform the valuable function of trapping particles of sediment.

The sump also provides a convenient location for the heater. Not only is it out of sight, it is protected from accidental damage during tank maintenance.

Accumulating biologically active debris is a further benefit of the sump. Fine particulate matter accumulates on the bottom. Thus, it can easily be siphoned out. Placing one or more baffles between the inlet side and the outlet side of the sump enhances debris collection. While the debris would be unsightly if left in the display aquarium, it is teeming with beneficial microorganisms that break down and recycle wastes, resulting in improved water quality. Saltwater enthusiasts sometimes refer to this material as magic mud in recognition of its benefits to the aquarium's ecology.

The sump can facilitate changing water, as well. By installing valves in the return line from the pump to the display tank, water can be directed either into the tank or to an external drain line. To operate, first, shut down the pump. Close the valve leading to the aquarium and open the drain valve. Turn the pump back on. When the sump is nearly empty, shut off the pump and switch the valves back to their operating positions. Refill the sump with replacement water, and then restart the pump to complete the process.

The capacity of the sump must be such that all the water in the plumbing system can drain into it without overflowing in the event of a power outage or pump failure. I suggest a sump of half the capacity of the main tank, although a much smaller sump may be enough to contain the water. Having a bigger sump makes working in it easier, increases the total water volume in the system, and minimizes any risk of overflow.

Some aquarists worry about designs that require drilling a hole in the bottom of the tank. Although a catastrophic leak is unlikely with proper workmanship, leaks do occasionally happen. You can have a truly fail-safe system by locating the sump above the display tank. In this case, the sump is called a header tank. A submersible pump located inside the display tank transfers water to the header tank. Near the top of the header tank, a drain hole allows water to escape, falling into the main tank through a connecting pipe. If the pump fails, a small amount of water drains into the display tank, and that's that. This arrangement presents additional engineering challenges, such as supporting the weight of the elevated header tank and making allowances for the lighting equipment, which must be located below the header tank and above the display tank.

Another way to eliminate the drain hole in the bottom of the display tank is to install a siphon to drain water to the sump. Unfortunately, this arrangement may also be vulnerable to problems. If the pump stops operating, water will continue to siphon into the sump until the main tank drains below the intake point. You can

82 Saltwater Aquarium Models

82 Saltwater Aquarium Models easily avoid such a disaster by drilling a small hole in the siphon just below the desired water level in the display aquarium. When water drains to a point below this siphon break, air rushes into the pipe and the siphoning stops.

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