Thaw fish and clams, if frozen. Rinse fish. Cut fish into 1-inch pieces. Set fish aside.
In a large bowl combine 2 quarts of the water and 3 tablespoons of the salt; add clams. Let clams soak for 15 minutes; drain and rinse. Discard the soaking water. Repeat soaking, draining, and rinsing clams two more times with the remaining water and salt.
For stew, in a large saucepan or Dutch oven combine potatoes, corn, undrained tomatoes, chicken or vegetable broth, thyme, garlic powder, and pepper. Bring mixture to boiling; reduce heat. Cover and simmer for 10 minutes.
Add fish and clams. Cover and cook for 4 to 5 minutes or until fish flakes easily with a fork and clams open. Spoon into bowls. Sprinkle with fresh parsley or thyme. Makes 4 servings.