Combing the olive oil, lime juice, chili powder, and cayenne pepper in a small bowl and whisk to combine.
Place the fish in a shallow dish (like a glass cake pan). Pour the marinade over the fish. Sprinkle the jalapeno and cilantro over it and let it marinate for about 20 minutes.
Grill over medium high heat for about 5 minutes per side. Actual cooking time will depend on the thickness of your fillet. The fish is done when it can easily be flaked with a fork.