Fish is my favorite grilling food.
If cooking filets, I like a combination of olive oil, white balsamic vinegar, and dried herbs of your preference (I like a Mediterranean blend). The vinegar, being acid, breaks the bond of the protein molecules, allowing them to separate slightly. This allows the oil to penetrate resulting in a moister fish. You can just as easily use lemon or lime for an acid. If lime, try putting a tablespoon of tequila in the mix.
Before you put the fish on, be sure to rub the hot grill several times (I do five) with an oil soaked paper towel.
When turning the fish, use a thin spatula, or better yet a couple of fish spatulas. The fish is ready to turn when you lift the edge gently with the spatula and find it releases easily. If cooking a skin on filet, first cook the skin side up for a couple minutes and then turn skin side down for the remainder.
You can determine how done a thick filet is by looking at the edge.
Have fun.