Mandarin dragonets are very difficult and generally only recommended for tanks at least 75-50 gallons. They eat very slowly and continuously throughout the day. Even if you train them to eat frozen or pellets, they still need an ample pod population to be able to survive long term. Newer tanks don't have adequate pod populations.
People do keep mandarins in smaller tanks, but they generally only survive a few years at most. Wild caught mandarins need to be trained to eat frozen foods, a process that could take weeks or months, and they need pods to eat in the meantime. Once trained to eat frozen, they also need to be fed in smaller tanks at least twice a day with all the pumps and filtration totally off. This allows the food to sit on the bottom where the mandarin can get it (no shrimp or hermits in the tank as they will eat all the food before the mandarin has a chance). Not only does this affect your water quality, it's a serious PIA when you go on vacation and you have to find someone willing and able to follow the regimen.
If you do decide to go for it despite all the warnings against it, ORA has tank raised mandarins that are already trained to eat frozen foods. Give Matt at Aqua Exotic a call. He orders ORA fish on occasion and can get it for you. It will probably be more like $50-$60, but it's worth it to get one that already knows what frozen food is.