you can usually buy purina fish food (a blue tilapia supplier recommended purina aquamax 5d09 from a regular purina distributor - i just asked a pet food store to order a bag for me (50lb=$26) - and will be picking it up tonight
you may be able to get by with rabbit, dog and cat food and duckweed.. buy why not use the appropriate food?
Yes louis, good idea...provided your water is clean enough as in snow melt or spring water, otherwise you might think of raising another type of fish for a multi trophic AP system.