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.