If your aquarium does not have a built in skimmer (normally in a back corner) you will need a hang on the back pre-filter skimmer. These filters have a cup with slats cut into the top of it that sets inside the aquarium. The skimmer cup helps prevent fish from being filtered out of the aquarium. When the pre-filter is working correctly the water going into the skimmer cup will fall into the cup causing the surface of the aquarium and any proteins collected at the surface to be skimmed off. The surface of the water in the skimmer cup should be a minimum of 3/4 to 1 inch (2 to 2.5 cm) below the aquarium water surface for optimal skimming.
All sump filtration systems operate very similarly. When setting up the system you must not overfill the sump, otherwise, if the power cuts out, you will end up with water over flowing onto the floor.
You can figure out the maximum operating water level height of a sump by cutting the power to the pump, and allowing the water from the main display to drain for about 5 minutes. Fill the sump with tap water to a point just below the point where water would overflow. Start the pump back up and allow the filtration system to run for about 5 minutes. Mark the water level on the sump with tape or a permanent marker. The mark on the sump will be the absolute maximum operating water level. Minimum operating water level in the sump will be the point where the pump starts drawing air.
Sump style filtration systems do evaporate a lot of water and will need to be topped off frequently.
Trickle (a.k.a. Wet/Dry) filters incorporate particulate, biological, and sometimes chemical filtration in their design. The water is first passed through a pre-filter (particulate filter) either in a corner of the aquarium, hang on the back of the aquarium, or top of the filter. The basic principal behind the trickle filter is to pass a trickle of filtered water over a media (plastic balls, plastic cubes, double layer spiral (DLS), or PVC) that serves as a surface area for beneficial bacteria.
Trickle filters oxygenate the water as it cascades through the biological media. Most trickle filters use a drip tray to distribute water over the bio-media. In most models you can add a heater to the sump, removing it from the aquarium.
Trickle filters come in several shapes and sizes. Some trickle filters are even built into the back of acrylic aquariums. A trickle filter is typically hidden inside the aquarium cabinet. The installation process and access to the filter must be considered when selecting a cabinet and filter.