Heat the grill to medium-high. Spray a large piece of aluminum with nonstick cooking spray or coat with olive oil, and turn it up along the edges. Cut the lemons and limes in half and squeeze all of the juice into a large measuring cup, being careful to remove any seeds. Peel and chop the garlic. Then, whisk the juices, garlic, and pepper together. Pour about one-quarter of the mixture into the aluminum foil; place the fish on top; and then pour the rest of the liquid over the fish. Loosely wrap it up, folding over the foil to keep the moisture in while grilling but with enough openings for the juice to mostly evaporate. Grill for about 5 minutes on one side and then open the foil to flip the fish once mid-way through, for a total of 10 to 15 minutes until cooked through (time will vary depending on grill temperature and thickness of the fish).