Combine the zests, juices, jalapeño and coriander in a bowl to form the base. Stir to mix well. Season the fish with salt and let sit at room temperature for 5 minutes. Add the fish to the ceviche base and stir, making sure all the fish is covered by liquid. Cover and refrigerate for at least 2 hours, stirring occasionally. Stir in the grapefruit segments and arils; add cilantro and scallions to taste.