Fresh albacore tuna is most used for this soup, but yellowfin tuna works well, too. Whichever variety you choose, look for very fresh, firm tuna with a clean smell and vibrant color, since its flavor really comes through in the finished dish. Aji para seco is a spice blend made with paprika and annatto (achiote) and is a quintessential seasoning in this soup, adding warm, earthy flavor and a rich reddish color. Look for it at Latin supermarkets or online. Yuca (cassava) has a tough, fibrous core running through the center that can remain stringy and unpleasant even after cooking. If you notice that your frozen yuca hasn’t been cored, slice each piece lengthwise and remove the firm, woody core before cooking for the most tender, even texture.