The psychedelic dragonet
(Synchiropus splendidus) is one of my favorite fishes. When I set up my current aquarium, it was with this fish primarily in mind. The information at the time recommended a large aquarium with a large amount of mature, live rock. A refugium was also highly recommended.
I have kept my female mandarin fat and "happy" for years in my reef tank. She was one of only a handful of fish that survived the "incident" earlier this year.
However, I added a red scooter dragonet (Synchiropus stellatus) to my aquarium a few years ago. Both fish appeared to do fine for the first six to eight months. But the smaller, scooter dragonette eventually died after about a year. My only conclusion was that my 120 gallon aquarium was just two small to adequately feed two fish with such a specific diet.
Anyway, that's my take - and that's why I think a 140 gallon, mature aquarium would make a great environment for a single dragonet.