Cut the fish into ½-inch (1cm) pieces. In an earthenware or glass bowl, combine fish, chopped garlic, aji pepper, cilantro and lime juice. Season with salt.
Cook sweet potatoes in a large pot of cold salted water until they are soft, around 25 minutes. Peel them and cut 1-inch (1cm) slices.
Add corn to the boiling water and cook for about 8 to 10 minutes. Cut into 2-inch (5cm) sections.