I would get yourself some Nutramar OVA and make sure you pick out a Mandarin who is actively hunting for food. I've been able to train every mandarin who hunts for food. I emphasize that because I've definitely been shipped Mandarin's or picked out ones who were goners from the get go from stress. They lived in a 20g long no sump and a 27g with 10g sump and fed OVA once a day. Had this pair for a 2 to 3 years before I lost them to a power outage.
I used them for about a year straight. The only problem I ran into was making the mistake of using a digital timer. One time it stuck in the on position for my alk and my tank looked like someone dumped a gallon of milk into my tank.