Are you going to have fish be sentient? Are all animals sentient Disney-style? If you are trying to make an at all coherent world, I'd just ditch the sentient fish part. Otherwise, I will honestly never read this because I won't be able to get over the horror of billions of sentient death just constantly. MOR!Harry panic about snakes right there. That is a really, really, weird world where humans haven't noticed as well. Fish are really, really, stupid. Hence we eat them en masse before we even started farming.