In a large pot, melt butter and add flour. Cook roux until nutty aroma and add onion, green pepper, garlic and crushed red pepper until mixture is tender, about 3-4 minutes. Add water and bring to a simmer.
Pour in tomato sauce, clam juice, Cajun seasoning and mustard. Then sprinkle with paprika, salt and pepper (to taste). Add Collard Greens, cover and simmer until desired tenderness (about 15 minutes). Cut fish into 2 inch cubes and cook until flakey (about 5 minutes).