the smaller split tanks, just use an airstone and have a single hole spray bar that runs above the tanks. The ends of the tank have submersible pumps attached to pvc to suck and create movement in the water. It probably goes into a huge central sump type filtration on each setup, but who knows.
SOme of the other setups are just sponge filters with the same set up as above. Only difference is the tanks are a bit bigger and each contain their own submersible pump.
Water movement is the key to clean tanks imo. Over filtration is always good.