A simple corner filter. This filter works on the same principles as commercially available corner filters, a rising stream of air bubbles draw water up with them this water is pulled through the filter bed first. The layers of gravel act as host surfaces for bacteria that preform biological filtration as does the filter wool which also acts a fine mechanical filtration. The top layer of gravel also acts as coarse mechanical filtration stopping large lumps of detritus reaching the filter wool. It came about as I dropped and broke the plastic case of a comercial one and I've been using these ever since (about 15years) in tanks of up to 150 litres
Step 1: Materials
a terracotta flower pot about half the hight of your aquarium,
a length of 22mm (3/4") pvc overflow pipe, about 2/3 the depth of your aquarium, a cylindrical air stone, air line, gravel, filter wool. activated carbon if desired this will then required a small section of fine mesh I normally use old net curtains.
Step 2: The lift tube
a section of 22mm pvc water overflow pipe is cut to about 2/3 the depth of the aquarium using either a hack saw and clean up the burs with a craft knife or just cut with the craft knife. perforate one end of the tube I used a 3mm drill they could just as easily be made with a hot nail or a soldering iron the highest hole must not be higher than the hight of your chosen air stone. The holes need to be evenly spaced and the more the better I normally manage about 30.
Step 3: Assembly
attach the airstone to the airline and pass the airline through the hole in the base of the flower pot and draw the stone down to the bottom of the pot. Place the lift tube over the airstone and hold in place whilst filling around it with gravel until the gravel covers the holes in the lift tube. If using activated carbon put about 12mm (1/2") layer in next I like to put a mesh separator in (it makes removing the carbon easier later) I use old net curtain material. Cut a square about twice the size of the top of the pot cut a cross in the middle and push it over the lift tube pack the carbon on top of it. Next pack in a layer of polyester filter wool stop about 12mm (1/2") from the top. fill up with gravel.
Step 4: Install in the aquarium
put about at least 12mm of gravel on the bottom of the aquarium so the airline doesn't get squashed position the pot in a back corner of the aquarium pack more gravel around the outside to about 1/4 to 1/3 of its hight connect the air and let it bubble away.