Place fish, shrimp and oysters in a bowl, squeeze lemon juice over, season liberally with salt and pepper and set bowl aside. In a heavy-duty large saucepan (preferably cast iron), heat oil over medium-high heat. Add flour a little at a time and stir constantly until medium brown in color. Reduce heat to medium-low and add onions, peppers, celery and garlic and cook for 7 minutes, stirring often. Reduce heat to low and stir in remaining ingredients and juice from fish, shrimp and oysters. Cook for an additional 10 minutes. Adjust seasonings with salt and freshly ground black pepper.