I made a DIY sponge filter for my hospital/fry tank.
All you need is some PVC
pipe, fittings, and cement, a large car wash sponge, an air pimp, and some airline. I also used a thermometer holder to prevent it it from floating. Others have used something to weigh it down, but I find the suction cup method to work just as well. Airstone is optional...I decided to use an airline shoved down the tube...
Here is everything you need (minus the airpump and airline):
I cut the sponge in half and drilled a hole for for the PVC
Here is what the PVC
pipe and fittings look like fully assembled (prior to glueing):
Completed sponge filter assembly...notice the hole drilled into the 45* piece for the airline:
Here it is in action: