Heart Scales can be used to teach your Pokemon moves you forgot or never learned at the Move Relearner in Dendemille Town. To obtain Heart Scales in Pokemon X and Y you need the Old Rod from Ambrett Town in the Aquarium by the Magikarp Statue and TM46, Thief, from Cassius's house in Camphier Town. Use the Old Rod on the Route 8 - Muraille Coast to battle Luvdics who have a 50% chance of carrying a Heart Scale. Keep fishing for an infinite supply of Heart Scales. Switch spots if Luvdisc doesn't take the bait.