I hate to give you bad news but your chances of keeping this mandarin are quite low. If you just recently set up and seeded the tank there's no way it will sustain the fish long term. As another stated, mandarins are best kept in large, established tanks with a sustainable pod population and little to no competition for said pods. If this is a wild caught fish it will likely be difficult if not impossible to get it to accept other foods. Trying to buy live pods is impractical as well unless you plan on doing so every other day. These fish appear to eat an average of 1 pod every 3-5 seconds for most of the day. Even captive bred individuals do best in established systems. Mandarins are one fish that's best kept by experienced aquarist with proper, established setups only. If you don't see undeniable evidence that this fish is accepting other foods within the next few days then for the sake of the fish your best bet is to return it if possible. The sad fate of most mandarins is slow starvation. Couple that with the fact that very many are half starved or worse by the time you receive them.
