I think the way the claim is phrased is misleading. Looking at the article, the author isn't claiming that fish is unhealthy, but rather that it's difficult to confirm that fish haven't been drugged with harmful chemicals or that they don't contain mercury. In actuality, fish cna be good for you, but there could be contaminants that have adverse affects. So it's not that eating fish is really bad for you, but actually that fish are often contaminated with harmful chemicals that are bad for you.