Stephen 44 needs some ideas for his new QT tank so I threw some spare parts together for him to see.
Fluid Bed Filters are one of the best Bio Filters we can build. It provides a tremendous amount of surface area for the bacteria to colonize and helps to saturate the water with oxygen. They are also self cleaning. Just fire it up and forget it.
With a hole saw start by drilling a 1 3/4 inch hole in the center bottom of the barrel. Then drill two 3 1/2 inch holes in the sides of the barrel. The inlet hole goes 9 inches up from the bottom of the barrel to the center of the hole. The outlet hole I put on the opposite side but it can go anywhere. The center of this outlet hole is 6 inches down from the top of the barrel.
Using 3M 5200 polyurethane adhesive install a 3/4 inch threaded bulkhead fitting in the bottom hole. Then after the adhesive is dry, screw in a 9 inch air diffuser. When you set this filter up put a 3/4 inch 90 in the bottom of the bulkhead fitting then another 3/4 inch 90 pointing up along side of the barrel. Reduce this to 1/2 inch and run the 1/2 inch PVC up to your air pump. I use the AV 50s.
Take 3 inch elect. conduit male and female adapters and screw them tight in the inlet and outlet holes. Use your 3M 5200 here also. Use elect.. conduit instead of PVC or ABS because the threads are straight cut and tapered pipe threads, so they will screw together tight.