In a medium pot, combine the whitefish, bok choy, tomatoes and radish. Stir together the tamarind paste and water; pour into the pot. Toss in the chilli peppers if using. Bring to a boil and cook for 5 minutes, or just until the fish is cooked through. Even frozen fish will cook in less than 10 minutes. Do not over cook or else the fish will fall apart. Ladle into bowls to serve.