Rinse fish and/or scallops; pat dry with paper towels. If using fish, cut into 2-inch pieces. Set aside.
In a 4- to 6-quart Dutch oven cook sliced fennel in hot oil over medium heat about 10 minutes or until tender, stirring occasionally. Add garlic; cook and stir for 1 minute more.
Add tomatoes, stock, dried oregano (if using), and, if desired, anise seeds; bring to boiling. Stir in fish and/or scallops. Return to boiling; reduce heat. Simmer, uncovered, for 6 to 8 minutes or until fish flakes easily when tested with a fork and/or scallops are opaque. Season to taste with salt and pepper. Stir in fresh oregano (if using). Garnish each serving with fennel leaves.