Well you can't blame people for thinking that cats love fish it's a popular belief and a lot of them actually do love fish. The scent drives them crazy and to some not so much.
It's quite common to think that way and people mean good. When they give their cats fish they do it wif wove
as in spoiling them. With time we all start learning more and more things about nutrition and many other subjects.
It's all true about the metals and the unrinary problems though.
As a curious story there's a small island in Mexico where a long time ago someone brought cats. As the decades went by colonies of wild (feral) cats developed in there and they fish! The literally fish. It's amazing, they catch and eat fish. They have healthy coats, lots of proteins. That of course doesn't discard other possible issues.
And well... That however is a very specific case because as in the post quoted above cats started out hunting mice and small pray. These guys in the island just adapted to what was most plentiful and available in their habitat.