Preheat oven to 425 degrees F.
Season yellowtail with orange juice, olive oil, orange zest, chipotle powder, paprika, cumin, garlic powder, dried oregano, salt, black pepper, and let sit for 20 minutes.
Meanwhile, make the salsa. Combine all ingredients including orange segments, grapefruit segments, habanero pepper, cherry tomatoes, red onion, juice of ½ lime, cilantro, and olive oil in a large bowl. Season with salt and pepper and let sit for 30 minutes.
Bake yellowtail for 12-15 minutes or until flesh flakes easily. Remove from oven and let rest for 5 minutes. Flake fish into smaller chunks.
Assemble tostadas. Spread guacamole on tostada shells and top with fish. Top with salsa and sprinkle cotija on top. Serve immediately.