Another fish tip, just for shits. Probably doesn't apply to the mahi mahi.
But, if you are grilling an oily fish like lake trout, or even salmon, fillet the fish and then lay it on a piece of brown paper grocery bag. Trim the paper around the outline of the fillet, then throw the whole thing on the grill. The paper will suck up the oil but won't catch on fire. (Doing this on foil would just allow the oil and grease to pool around the meat.) Once it's done, the skin will stick to the paper and the meat will pull away. Money. I probably wouldn't eat lakers if I didn't know this method.