To make grilled fish tacos, first heat your grill for medium-high direct heat (425-450 degrees).
In wide, flat bowl, combine oil, chili powder, cumin, jalapeño and salt, and mix well.
Cut each fillet down its centerline, dividing it into thicker and thinner sections. Toss fish pieces in chili oil mixture, coating them evenly, and set aside in mixture for 15 minutes.
Set a grill screen on grill and coat with oil. Place the thicker fish pieces on screen and cook, turning once, until opaque all around the edges, about 3 minutes per side.
Cook thinner pieces the same way, but for only about 2 minutes per side. Transfer fish to platter and keep warm. Remove grill screen.
Lightly spray tortillas on both sides with cooking spray and brown directly over the fire, turning once, until grill-marked and flexible, about 20 seconds per side.
Wrap tortillas in clean kitchen towel to keep warm.