In a saucepan, sweat the red pepper flakes in the olive oil for about 30 seconds.
Add the scallions, fennel, and green pepper, and continue to sweat for about 3 to 4 minutes over moderate heat.
Add the tomatoes, including the juice, and bring to a boil.
Simmer for about 8 minutes until all vegetables are tender.
Combine the halved shrimp and diced tilapia with the scallops in a bowl and season with salt and pepper; mix very carefully.
Add the seasoned seafood to the simmering tomatoes, bring to a near boil, and cook gently for 3 to 4 minutes.
Add the crabmeat and basil, stir very carefully, and heat through.
Divide stew into 4 equal portions (about 2 cups per serving), garnish with basil leaves, and serve hot.