In a 12 to 14-inch saute pan, heat the oil until almost smoking. Add the onion, garlic, celery, and potato and saute 8 to 10 minutes, until golden brown. Add the chile flakes, olives, capers, and tomatoes, and cook 7 to 8 minutes, until the tomatoes have fallen apart. Add the water and bring to a boil. Season the fish well with salt and pepper, place it in the pan and cook, covered, for 8 to 10 minutes. Sprinkle with parsley and serve.