Ten Penn-Plax undergravel filters; two are ~30" x 18" (for a 5-foot 100g tank) and eight are ~24" x 12" (for four 4-foot 55-g tanks). I had these in use until I rebuilt my fish room about ten years ago and upgraded everything. These are an old-school technology, but can still be useful in certain situations. For example, set them up with lift tubes and powerheads in a tank with a reasonable amount of substrate, and you have an abundant source of biological filtration to supplement whatever external filters you might be using. Gravel-vac every now and then, and you are good to go for long periods of time. 
