I wouldn't jump to that conclusion. Fish are very much aware of their surroundings. My discus will watch me from 5 meters away. Gold fish were taught to "drive" a mobilized aquarium to specific locations within a room.
When I lived in Seattle, there was an octopus at the local sea aquarium that would climb out of its tank at night, crawl across the floor, climb into a another tank, eat one of the fish in that tank, then return to its own tank afterwards. This went on for days, and the keepers at the aquarium couldn't understand why fish kept disappearing. It wasn't until they installed cameras that they discovered what was happening.
I agree fish are conscious to a degree but a lot of the concepts that we judge ethical or not are just beyond their comprehension. I think the longer lives, lack of predation etc definitely weighs into the subject too as those are things fish can probably comprehend - though not even sure they would be able to comprehend age as time is quite an advanced level of thought...
I don't think comparing an Octopus to a Discus is truly accurate though, Octopus are so unique in comparison to anything else on the planet we can't assume if a Cichlid had tentacles they would behave the same. Some people actually wonder if Octopus originated on Earth, but thats a whole other story haha!
Wills