Fish can have really bad things in it for you, especially when it was on a farm. Fish farms are way too overcrowded, and most if not all of the fish have some sort of infection or disease. Unless you want to buy really expensive fish, you are putting those toxins in your body. That doesn't make you much healthier.
With that said, Chickens could be the same case. But it is much cheaper to buy good Chicken then it is to buy good fish. It is a lean protein that is very good for you.
A mix of both. You'd need both healthy fish and chicken, because I believe chicken provides iron? I'm not sure, and fish is important to your diet as well.
