Heat the oil in a 4- to 5-qt pot over medium heat. Add the onion and carrots and cook, covered, stirring occasionally, until onions are tender, 6 to 8 minutes.
Stir in the garlic, fennel seeds (if using), thyme, and ½ tsp each salt and pepper and cook, stirring, for 1 minute.
Stir in the broth, potatoes and 2 1/4 cups water and bring to a boil. Reduce heat and simmer, covered, until the potatoes are tender, 8 to 10 minutes.
Add the tomatoes (and their juices) and tomato sauce and return to a simmer. Stir in the cod and simmer until opaque throughout, 4 to 5 minutes.