Processing the feed for the fish is a complete system unto itself.
In addition to the bits of plant matter carried down through the conveyance system by irrigation water, vegetable matter that is left over after the mature plants have been harvested can be used as a food source. This source of food needs to be broken into small enough pieces for the various sizes of fish to eat.
A mechanism much like a meat grinder can be used to cut the stalks and root mats into palatable chunks to be fed to the hungry fish. Excess vegetable matter is fed into a bin that narrows into an auger that cuts and moves the bits through a sizing screen or grate. Have you ever seen those home french fry cutters? Something like that would work grate (excuse the pun).
The sized bits would be plopped into the tank with the appropriately sized fish in it. This system makes the most efficient use of the nutrients in this almost closed loop system. Additional sources of vegetation would be welcome into the system, especially proteins.
Amounts to be fed and frequency (or speed) of feeding can be controlled electronically through the appropriate hardware and software combination. The extra veg fed into the bin would have to be moved by hand (for now, but not forever). I will try to get some pictures up for all my logs...