ANSWER: Stefan, your dad encountered the rare and aptly named gorgeous swallowtail, Meganthias natalensis. These fish are actually close relatives of groupers, and are placed by taxonomists in the subfamily Anthiinae, a group of relatively small, colorful fishes known as fairy basslets, within the family Serranidae. Gorgeous swallowtails are the largest of the fairy basslets, growing to more than 16 inches long (to the tail fork, so excluding the spectacularly long, slender lobes of the caudal fin). This species is known to occur only in the Indian Ocean, along coasts of South Africa, Mozambique, Kenya, the Seychelles and Reunion Island, and as far east as the Andaman Sea near Thailand. They frequent deep reefs, usually between depths of 250 to 600 feet, so compared to other captures, this fish was taken in relatively shallow water. Like other serranids, gorgeous swallowtails are a predatory species, feeding mainly on other fishes. Given that some skippers in the Seychelles report seeing only one or two gorgeous swallowtails throughout their whole career, you and your dad certainly had luck on your side that day.
— Ben Diggles