I reckon, eating fish as a main source of protein is so much better than eating commercially produced chicken meat, beef, mutton, lamb etc. We would never know what have actually gone into those animals before being slaughtered. If there is any growth hormone being injected to make them grow much faster, or appear much healthier than they really are.
Fishes, on the other hand, are caught directly from their habitat, minimising the risk of any contamination with unnecessary chemicals.
I think, to eat healthy is to eat more naturally produced food, as much as possible. I have a suspicion that many modern diseases stem from these unnatural food.