To simplify The amount of feed needed to sustain and grow an octpus is three times the weight of the animal itself where feed is delicious protein.
The small problem is that the taste of this fishes is plain awful and _nobody_wants_to_buy_it_.
There are plenty of cow breams in the wild, but you rarely will find it in your fish shop. And if you eventually find it, will be a really cheap product. Try it. Most people will not buy it twice.
Investors aren't stupid. They risk a lot of their money and put incredible efforts to breed carnivorous fishes for a logical reason. Everybody loves its taste and are willing to pay good money for it!.
So, in practice it’s more a question of feed stock ratios. Salmon for example can be fed a low percentage of soybeans to lower costs. However, as you can’t replace even 50% of the feed stock with plants the overall argument is still valid.