Also the non plant parts of it could be fairly efficient too. e.g see black soldier fly farming for fish feed. Not that that is a 100% answer for all feed for seafood farming, but interesting options abound.

Personally though, Octopi are too smart for me to eat. I'll always pass on it now.

Whatever happened to the Roach farming as a viable option for quality protein farming? I could see a flow like algae/hemp -> roach -> fish -> human

Soldier fly leaks from farms are more palatable than Roach leaks?

This seems to be a very generalized statement when there is a lot of variation to consider. There are 33,000 fish species, some being specialized feeders of algae, to 100% live fish, all designed for processing each food differently. Are you speaking on a specific fish that is commonly "farmed"? I could see Salmon needing this 20kg and not even contain the quality fats they get in the wild, resulting in a lower quality food on top of being more impactful to the environment.