Any fish worth putting on the barbeque, 10 minutes TOTAL cooking time per inch of thickness tops.

Pull it off the heat med rare, let rest for a few minutes, and it physiologically finishes cooking to a juicy medium.... just barely becoming opaque in the middle.

If the fish has been frozen, the cells are dead, and the meat will not readily "finish" as described... tends to stay underdone in the middle. If you cook the middle thoroughly the rest is overcooked. Just one of the unavoidable culinary perils of freezing perfectly good FRESH fish. Better to keep the pieces thinner so its easier to cook it more evenly full thickness.