Can it be done sustainably? Can a complete ecosystem be established where the feeder fish don't have to be harvested from elsewhere?
Though I'm struggling to think of even just one example of a human-created ecosystem designed to substitute for a natural one that's been at least as sustainable as those existing in nature.
Sun -> algae -> small fish -> medium fish -> fish we eat.
Very doable, the problem comes in dealing with the waste.
As I understand it, after a few years the floor of the fish farm is basically a solid toxic layer of fish food and fish shit, and it becomes unsuitable for farming. That's not sustainable.
But as things stand right now, the process as implemented is completely unsustainable.