Remove husks and silks from corn. Coat 3 large pieces of heavy-duty aluminum foil with cooking spray. Wrap each ear in a piece of foil. Twist foil at each end.
Bake at 500 for 20 minutes.
Remove from oven and let cool slightly.
Cut corn from cob, and place in a medium bowl.
Add tomato, onion, 1/2 cup cilantro, and next 3 ingredients; stir well.
Place 1 fish fillet on each of 4 additional large pieces of heavy-duty aluminum foil coated with cooking spray.
Combine yogurt, garlic powder, and salt; spread evenly on both sides of fillets.