If the Mandarin is super tiny you can start with Hikari Mysis, Otherwise PE Mysis is absolutely the best way to go. I've had several Mandarins and all were trained to PE Mysis. It keeps them fat and healthy. I've had 2 breeding pairs that would do the mating dance every night. That only happens when they're very well fed. The best way to do it is to use a coral feeding tube, that looks like a turkey baster, and squirt some in a low flow place near them. If you have other fish then shoot some higher in the tank for them so the Mandarin doesn't have to compete. They train pretty quick and will learn a specific feeding spot. once they're trained, usually 2 weeks tops, they start chasing the Mysis down. That's the best, and really only way, to have long term success with Mandarins.