From this article I just read, albeit an opinionated one, says that fish is actually not a good idea to be part of the cat’s main meal.
I am not so concerned about the toxins in fish, because it is present in many seafood sources, and is so even for humans. I am more concerned that it is allergies-causing. While fish is a low allergen protein source for dogs, it is a high allergen source for cats.
Right now the only fish that is in our cats diet are in their dry food, and the occasional tuna and salmon sources in their canned food.
For some strange reason, our cats no longer love Fish4Cats Salmon Mousse which I occasionally buy even though it is expensive. Our cat supply of canned food is running low now and I think I will be choosing more chicken and other poultry or from hooved animals instead of fish. The cats love chicken anyway!