What is the best way to reheat leftover grilled fish?
If what you want is a piece of fish with the same texture and juiciness as fresh-off-the-grill, yes it is a lost cause.

My preferred method for reheating leftover fish is to break it up and use it in another application. Bouilliabaisse is excellent for leftover bits, any fish stew really. Also fish tacos.

Simply break up the fish and add to the stew in the last few minutes (to prevent overcooking), or toss briefly in a pan.