Preheat oven to 400°.
Bring first 3 ingredients to a simmer in a small saucepan (do not boil). Keep warm over low heat.
Peel the shrimp, leaving tails intact. Combine shrimp, grouper, and squid in a bowl; sprinkle with salt, let stand 5 minutes. Heat 1 tablespoon oil in a 13-inch paella pan or large skillet over medium-high heat. Add fish mixture, and saute for 1 minute (the mixture will not be completely done). Remove fish mixture and any liquid from pan, and place in a bowl.
Heat 2 tablespoons oil in pan over medium-high heat. Add bell peppers, onion, garlic, and paprika; sauté 3 minutes. Stir in tomato; cook 2 minutes. Stir in rice, and coat well. Add clam juice mixture; bring to a boil, and cook 3 minutes. Add reserved fish; cook 2 minutes. Add mussels to pan, nestling them into rice mixture.
Bake at 400° for 15 minutes or until shells open; discard any unopened shells. Remove from oven; cover and let stand 10 minutes.