Stir in the tomatoes and potatoes. Cover with the stock or water and bring to a boil. Turn the heat down to medium-low and simmer, stirring occasionally, until the potatoes are tender and the broth has thickened, about 15 minutes.
Season the fish with salt and pepper, nestle it into the tomato mixture, and cover with a lid. Simmer until the fish is tender but not falling apart, about 7 minutes.