Fish Puttanesca

1/2 pound spaghetti 2 tablespoons olive oil 2 tablespoons drained capers 1 tablespoon anchovy paste, or to taste 4 cloves garlic, peeled and thinly sliced 1 teaspoon crushed red pepper (optional) 1 (28 ounce) can whole, peeled San Marzano tomatoes 1/2 cup pitted and sliced Kalamata olives 2 tablespoons minced fresh Italian parsley salt and pepper to taste 4 (6 ounce) cod filets, or other mild white fish fresh Italian parsley sprigs for garnish (optional)