Terrible is a subjective term, but anyway although I agree with some of the stuff, I'm going to have to disagree with a few things here. This info is based off school research projects I have done.
Fish is generally good for you, however, if you have to much it can be harmful. For example if you had tuna every day within a year or two the mercury levels in your body would be above safe limits. If you continues consuming a tuna sandwich each day it would inevitably lead to mercury poisoning. But even if you're not a tuna fanatic, elevated levels of mercury over time could lead to health issues. Of course the exact timing would depend on body weight, amount consumed and other factors.
Certain GMO crops are designed to withstand higher levels of pesticides than traditional crops, allowing farmers to spray stronger pesticides without the crops dying. Then gradually the weeds adapt to the pesticides becoming immune. Often the way to counter this is to put even more pesticides on the crops, then the weeds get stronger and the cycle continues. The result is increased pesticides on the produce without significant gains in production over time. Eat one GMO product and it will likely do almost no harm, but consume chemicals over a lifetime and who knows what kind of damage it could do. I'm speaking in generalisations here, the amount of pesticide and effects vary depending on the company and type of produce. For example, pineapples even if sprayed with a lot of pesticide contain almost none when you eat it because of the amount of skin removed before eating.
For fatty foods it does depend on the type of fat. I once heard that the Maasai tribe live off only meat and some lived past 90 and therefore eating animal fat shouldn't cause increased cholesterol. Well I experimented with this and ate as much animal meat and fat as I wanted for about a year. When I took a blood test later my cholesterol level had significantly increased. Maybe the Maasai people bodies have adapted after generations of the diet, I am not sure, but if I continued with that died it probably would have lead to heart issues.