Add the thyme, bay leaf, parsley, garlic, tomato paste, tomatoes, saffron, cayenne pepper, potato, and salt. Add 2 containers of fish stock. Bring to a boil, reduce the heat to medium and simmer, covered, approximately 35 minutes. Once potatoes are almost cooked through, add in the fish. Remove soup from heat when fish is cooked through and potatoes are softened completely. This will take roughly 45 minutes.