We can grow all our own fish food.
We can feed the fish with manure. Read this research paper for more detail
We can grow duckweed so as to sequester Nitrogen from the air and create a rich staple diet for our fish. This is the entry point where Nitrogen enters our Aquaponics cycle, much like nitrogen fixers are used in Permaculture.
Here’s a nice idea to allow the duckweed to fall into the fishtank automatically – without work or tech.