The tank looks good!
Cee is probably refering to Mandarin's being notorious for perishing from starvation in newer tanks. This is mostly due to their diet consisting entirely of micro fauna (live copepods, mysis shrimp, and most likely amphipods). A newer tank rarely has established populations of these for the Mandarin's to subsist off of. They can be added manually and will help keep your tank clean as well as provide food for the Dragonet.