Fish, cereal, yogurt and other basic food can be very healthy, until it is corrupted in the hands of food manufacturers. The more food is processed, the unhealthier it becomes. The healthiest foods have no list of ingredients.
Picture a wholesome meal: lots of veggies, maybe some pastured meat or free-range eggs, lovingly cooked at home from scratch. Do a quick count of how many of your meals from the past week looked like that. Close to zero?