Melt the butter in a large Dutch oven over medium-high heat. Add the onions, celery and a big pinch of salt. Cook, stirring occasionally, until the vegetables start softening, about 7 minutes. Add the chicken broth, 2 cups water, the clam juice, potatoes, bay leaves and 3/4 teaspoon salt. Make an ?X? in the bottom of the chile pepper with a paring knife, exposing some of the seeds; add to the broth. Bring to a boil, then reduce to a steady simmer and cook until the potatoes are just tender, 13 to 15 minutes.
Season the fish with salt, add to the soup and simmer until starting to flake, about 5 minutes. Discard the chile pepper and bay leaves. Stir in the scallions. Divide the soup among bowls and top with parsley and pepper.