From The Aquarium Wiki
- Hamburg matting Filter (HMF)
This is a DIY basic air driven biological bacterial filter which takes up the entire side of an aquarium, usually the shortest side. It was invented in Germany and is designed to be a very cheap, robust and efficient filter for preventing fry, small fish, frogs or tadpoles from being trapped by the inlet of a normal filter.
It consists of an air-pump, air tubing, optional airstone, a non-return air valve, a small plastic or PVC tube 0.5" in diameter that has a right angle at the top and the overall height of the water level and an aquarium-safe block of foam about 1-2" thick that will fit tightly into the walls of the aquarium and be slightly taller than the water level.
With the PVC pipe with cut-out notches at one end standing on the floor of the aquarium, the right-angle end needs to be just above the planned water. At the right-angle drill a small hole for the air tubing and place inside a length of the tubing (optionally with a air stone such that the stone is at the bottom for reduced noise). Seal the hole where the air tubing comes out so it is air tight.
Take the foam and cut it to size so that it needs to be compressed to fit inside the walls of the aquarium. Some people cut out a shallow notch in the centre of the foam so that the PVC tubing rests within the foam for a tidier look.
Make sure the foam goes all the way to the floor and above the planned water level. Some people cut a notch at the top of the foam so that the PVC pipe rests on top for a tidier look.
This filter will have a huge surface area and should never clog in practise for about 2 years or more.