I feed the fish a regular pond fish food, that seems to do as well as any other food for them.
As to scalability, My system is really quite small. I've seen systems on youtube that have tanks that hold several thousand gallons of water and hundreds of fish. Of course, the growing beds are much larger also. The fish are usually tilapia, raised to eat, a double food system. People do aquaponics on a commercial scale in some of the big cities in the U.S.