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A wet/dry filter, also known as a trickle filter, provides mechanical, biological and optionally chemical filtration for an aquarium. The wet/dry filter is the preferred filter for many aquarists because of the handy sump it contains.
A wet/dry filter sits under the tank, normally inside the aquarium stand. There are many designs, but basically the water is drawn down from the aquarium where it enters a large rectangular container. The water first passes through a cleaning medium like a foam pad to pull particulate matter. It may then pass over chemical resins or activated carbon to adsorb organic pollutants. Next it enters a drip tray where it drops through air to gain maximum aeration before falling on to filter media below that is designed to house bacterial colonies that make up the biological filter. Once the water passes over the biological material, it enters a sump where it collects in a bath. A pump is placed in the bottom of the sump, and the water is returned to the tank at such a rate as to match the flow into the wet/dry filter. This regulates the level of the sump.
There are several commercially made wet/dry filters but many aquarists make these filters themselves so that they can create a larger sump area. The sump areas in commercial wet/dry filters, which are normally made of acrylic, are relatively small. A large sump can hold equipment like heater(s) and protein skimmers to give the tank a cleaner look. The sump is also handy for testing the water or adding chemicals, rather than going into the tank, which is usually covered with a canopy and lighting fixtures.
A large sump is also advantageous because it expands the surface area available for the tank, which improves oxygenation. This is especially critical with tanks that are heavily stocked.
Do-it-yourself (DIY) wet/dry filters can be made from Sterilite or Rubbermaid food-grade plastic tubs and PVC. This route is quite inexpensive and one can make extremely large wet/dry filters that virtually never need maintenance, except for occasional rinsing of the mechanical filter medium. One popular method is to use a 20 gallon (78 liter) tub for the filtration container, which feeds into a 50 gallon (189 liter) tub used for the sump. Many saltwater enthusiasts like to place live rock in the sump. Other equipment, like pH meters can also be kept here.
In short, a sump makes the aquarium water easily available to test, adjust, and monitor, and the general rule is, the larger the sump, the better. Like commercial wet/dry filters, DIY wet/dry filters can also be made of acrylic; some aquarists prefer this as plastic tubs can get brittle over time and might eventually need replacing, unlike acrylic.
Many plans for DIY wet/dry filters are available online. If you do use plastic tubs, make sure they are food-grade or have been used before in the application of an aquarium filter without problems. Some plastics can leach harmful chemicals into the water.
Wet/dry filters remain a staple choice for aquarists everywhere. Whether manufactured or DIY, a wet/dry filter will provide excellent filtration for years to come. And once you have the convenience of a sump, it will be hard to go back to anything else!