Should you decide on a glass tank, or have an existing glass tank, and do not want to have holes drilled into it there are External Over Flow systems available. These units consist of a surface skimmer box that sits down into the water allowing the surface water to be drawn off. As the water fills the skimmer box this External unit uses siphon tubes to draw the skimmed water out of the box, and then routes it over the edge of the tank and into a prefilter box that hangs on the back of the aquarium. Both the Internal and External Over Flows allow the waters organic skin layer to be drawn off, but the External requires much more attention and maintenance. Within the Overflow Box, or External Overflow, one places a filter media, such as a filter pad, to seperate or trap the large pieces of debri's that are drawn into the overflow system.
Mechanical filtration is simply a media that traps out particles but lets the water pass through.
It is important to remember that this pre-filter needs to be cleaned or replaced in time. For the Undergravel and Canister filter appraoch glass tanks have a long plastic strip at the top back edge of the tank that is easily trimmed to accomidate the inlet and outlets of canister filters, power filters, and power cords.