Make dark brown roux with oil and flour. Remove from heat and add onions, bell pepper, and celery, stirring constantly until cool. In 8 qt stock pot, heat water and add garlic powder, oyster juice, and roux. Bring to boil and simmer for 45 minutes. Add seafood and seasonings and simmer for 15 minutes. Add green onions, parsley, and tabasco and simmer for 10 minutes. Serve over rice topped with gumbo file.