Is it safe to eat fish that has been left out in the cold overnight?
I hate throwing away food, for me (and I stress "for me") I would use the "Smell By" test and then make sure it is cooked really well through.  Make a stew, curry or pasta source (something where it is cooked through and in small bits). BUT DO NOT sushi, light pan fry or similar of it.

If you are in doubt do not use it, you do not want a dose of something a week before holidays.

Other factors
Was the fish cleaned?
Was it in bag or one of those polystyrene (Styrofoam) boxes?
Was it in ice?