Place vinegar and sugar in a small saucepan; bring to a boil. Remove from heat. Place onion in a large bowl. Pour vinegar mixture over onion, tossing to coat; cool. Add peaches, grapefruit, mint, 1/4 teaspoon salt, and 1/4 teaspoon pepper to onion; toss gently.
Sprinkle fish with 1/2 teaspoon salt and 1/4 teaspoon pepper. Place fish on grill rack coated with cooking spray; grill 5 minutes on each side or until fish flakes easily when tested with a fork.