Place fish in a shallow bowl, and cover with 3 tablespoons juice, 1/2 teaspoon salt, and pepper. Cover and refrigerate 20 minutes.
Combine remaining 1 tablespoon juice, yogurt, sugar, 1/4 teaspoon salt, and chili powder. Cover and chill.
Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Add fish and marinade to pan. Cover and cook 8 minutes or until fish flakes easily when tested with a fork or until desired degree of doneness. Cut fish into 8 equal pieces.
Warm the corn tortillas according to package directions. Top each tortilla with about 2 ounces fish, 1/4 cup shredded cabbage, 2 tablespoons yogurt mixture, 2 avocado slices, and 1 1/2 teaspoons cilantro.