I do eat a lot of fish and have never needed to throw any away. Of course I am a little fussy about the fish I choose from a supermarket shelf. If the tuna looks rainbow coloured I don't buy it and it is all marked with a best before date which is usually a reasonable guide as to which I should buy.
But please don't get me wrong, I'm not saying that a supermarket is superiour to a fish shop. If there was a fishmonger in Forest Hill I would probably be in there most days.