Wash the rhubarb. Cut into 1-inch cubes. Place the water and the rhubarb in a large saucepan, and cook until very soft, about 15 to 20 minutes.
Add the tomato sauce, sugar, salt, and oil. Cook over low heat for 5 minutes. Add the fish and cook gently until tender, about 15 minutes. Taste the sauce and adjust the seasoning, if necessary. Add water if the sauce is too thick. Serve hot or cold.