Yes, I've seen QI too. Funny show, but wrong for the sake of entertainment. Fish are aquatic animals. The premise that if a category for animals isn't derived from biology that it therefore "isn't real" is simply and obviously wrong.

It would be like saying the culinary categories of fruit and vegetables aren't real because many of the foods categorized as vegetables are considered by biologists, on the context of discussions about biological rather than cooking, to be fruit. What we actually have is multiple systems of categorization which both have their place, but users of one are trying to claim absolute supremacy over the other, denying the validity of colloquial folk taxonomies even in casual contexts.