Thaw shrimp, scallops, and fish, if frozen. Peel and devein shrimp, leaving tails intact if desired. Rinse shrimp, scallops, and fish; pat dry with paper towels. Cut any large scallops in half. Cut fish into 1-inch pieces. Cover and chill until needed.
Scrub mussels under cold running water. Using your fingers, pull out and discard any beards visible between mussel shells. In an extra-large bowl combine 3 quarts water and 3 tablespoons salt. Add mussels; soak for 15 minutes. Drain and rinse thoroughly. Repeat two more times with fresh water and salt. Drain and rinse mussels thoroughly. Meanwhile, in a large saucepan cook potatoes in a large amount of boiling water for 10 to 15 minutes or just until tender. Drain; set aside.
Cut off and discard fennel stalks, reserving some of the feathery tops if desired. Remove any wilted outer layers from bulb; cut a thin slice from base of bulb. Cut bulb in half lengthwise; discard core. Finely chop fennel bulb. Snip the reserved fennel tops; set aside.
In a 5-quart Dutch oven cook and stir finely chopped fennel, onion, and garlic in hot oil over medium heat until tender. Carefully stir in broth, tomatoes, 1/2 teaspoon salt, lemon peel, saffron, and cayenne pepper. Bring to boiling; stir in shrimp, scallops, fish, mussels, and potatoes. Return to boiling; reduce heat. Simmer, covered, about 5 minutes or until mussels open. Discard any unopened mussels.
Top each serving with Garlic-Parmesan Croutons. If desired, garnish with the reserved fennel tops.
Preheat oven to 350 degrees F. Line a 15x10x1-inch baking pan with parchment paper. In a large bowl combine bread, olive oil, Parmesan cheese, and ground black pepper. Toss to combine. Spread cubes into prepared pan. Bake about 20 minutes or until golden, stirring once or twice. Cool in pan on wire rack.