Often going by the common name "gobies", the Dragonets actually occupy an adjacent Suborder (the Callionymoidei). "True" Gobioids vary from them in a few obvious ways: placement and shape of pelvic fins (anterior to the pectorals in Callionymids, posterior and suction-like in Gobioids), the opening of the gills (small apertures up behind the head in Callionymids, larger in gobiods) .The fourteen genera and about 186 species of the family Callionymidae are typified by small gill openings, by having broad, depressed heads, and scaleless bodies with two dorsal fins... Living on the bottom with a characteristic "scooting" type of locomotion.
The predominantly offered genus is Synchiropus. Synchiropus splendidus is the Blue, and Synchiropus picturatus, the Green or Spotted Mandarin. These two have one of the most dismal survival records of captive marines. Almost all perish within a month of wild capture... most often due to simple starvation.
The principal selection criteria for picking out healthy dragonets are their fullness of body and activity level. Suitable specimens should not be skinny, and should be out and about, investigating their environment. For sure you want to see the specimen/s eat.
Some species of mandarins offered in the trade are found on nothing other than sandy bottoms, but the Green/Spotted and Blue/Psychedelic species are almost always located in and amongst (mainly Acroporid) coral rubble, which they only venture out past sunset to feed and interact with their own kind.
As stated over and over, feeding, or rather a lack therein is THE common cause of loss of these animals. They spend many hours seeking out small live invertebrates living in/on live rock and substrates. If these are not present or otherwise supplied, you will see your mandarins sides sink in and its vigor wane. Live foods can be bought on a regular basis, cultured in separate vessels, in an attached fishless refugium. Starter cultures for these organisms can be purchased from companies that you can find on the Internet using the search terms: "live plankton fish food".
Do not fall into the trap of offering nothing but adult live brine shrimp, suffused with supplements (e.g. Selcon) or not. It's not unheard of that a dragonet will accept (with training) frozen/defrosted foods in place of live, but it is rare that non-live foods sustain them.
Dragonets are notably slimy fishes that are not as susceptible to external parasite infestations as other fishes. However, they are not immune, and are overly sensitive to copper compounds, other metal-based and formalin containing medications. They are best treated through environmental manipulation (hyposalinity, elevated temperature) should they show signs of such afflictions.
Whatever other writers have stated, Mandarins almost never accept enough of anything other than live foods that are omnipresent in their system to sustain themselves. A nutrient rich live rock reef tank, read that as one heavily populated with hard substrates, with substantial interstitial crustacean and worm, and other small sessile invertebrate life of about 100 gallons will support one individual. And this assumes you have no similar food-competing tankmates.
In the wild their food choices are principally small crustaceans and worms. You can culture these "incidentally" in a large main/display system with lots of substrate and/or live rock, but adding a live fishless refugium will go an immense distance in assuring your mandarin/s receive sufficient live food. These fishes cannot live on dried-prepared or frozen/defrosted, or chopped meaty foods.
Callionymids are very docile when it comes to competing for food or space and must need be kept with other very easygoing fish species or perish from harassment or lack of food. Also overly aggressive invertebrates should not be mixed with them. Dragonets are good reef aquarium specimens, leaving alone all desirable species, but may in turn be consumed by anemones, the large coral anemone (Amplexidiscus), or large crustaceans. Ideal tankmates include tube-mouthed fishes (seahorses, Pipefishes), small blennies, Jawfishes and gobies, Dartfishes, flasher and fairy wrasses. Larger wrasses, Dottybacks, goatfishes, most butterflyfishes, angels and puffers, triggers are definitely out.
Though seemingly defenseless in their slow swimming and "scooting" locomotion, this group of fishes is widely unpalatable… some evidence exists that their body slime is toxic or at least unpalatable. This and a prominent gill cover spine serves these fishes well as predator deterrents in most cases. A related note re this opercular spine; take care to scoop out mandarins with a bag or specimen container rather than a mesh net, as these can get fouled easily with the protuberance.
You may well see them kept in a group at a dealers but be warned, this is not a natural setting, and all species should be kept either one to a tank or as a single male with more than one female or a heterosexual pair. Otherwise, unless the tank is very large (hundreds of gallons) eventually you will see them fighting vigorously.
For species known, Callionymids are pelagic spawners that come out past sunset, "do a formalized dance" and release their gametes into the environment, where they float up into the moving upper water column via currents. Spawning occurs at about weekly intervals (Algosaibi 1983), eggs are released of a few hundred in number, approximately a half to one millimeter in diameter, that hatch out in about a day. Depending on species, young remain in the plankton from a week to forty days before settling down. Unfortunately no young have been raised to date.
Though far less frequently imported, males are easily distinguished from females by much longer (about three times the length) first dorsal fins, larger size and brighter coloration.
Some of the Few Dragonets Seen in the Trade:
Most dragonets are rather bland in coloration, a smattering of brown, black, yellows… but a few are spectacular "paisley" prints. All are comical in their behavior.
So, let's review. To successfully keep dragonets one needs a very peaceful, large reef system with lots of live rock and deep sand bed and/or such a system with a vibrant fishless refugium (highly recommended). Don't have this sort of set-up? Leave these fishes in the ocean.
Algosaibi, Farouk A. 1983. Spawning mandarin fish, Synchiropus splendidus (Herre). FAMA 5/83.
Bartelme, Terry D. 2001. Caring for a Mandarin. FAMA 6 & 11/01.
Carlson, Bruce A. 1983. The mandarin fish Synchiropus splendidus (Herre). FAMA 2/83.
Cuttriss, Alastair M. 2001. The Mandarin dragonet. FAMA 4/01.
Debelius, Helmut. 1987. Mandarin dragonets in the marine aquarium; spawning at night. Today's Aquarium-Aquarium Heute 1/87.
Delbeek, J. Charles. 1989. The mandarin fish: Synchiropus spendidus (Herre). SeaScope Fall 89.
Kurtz, Jeff. 2000. Synchiropus: Dragonets with style. FAMA 5/00.
Lang, Tom. 1998. Care and feeding of the Mandarinfish (Synchiropus splendidus). Aquarium Frontiers 1/98.
Mayland, Hans J. 1975. The Mandarin. Marine Aquarist 6:3/75.
Michael, Scott W. 2000. The dragonets. Beautiful, but not for everyone. AFM 9/00.
Michael, Scott W. 2001. The very common problem of feeding mandarin dragonets. AFM 11/01.
Sprung, Julian. 1994. "Reef Notes". FAMA 8/94.
Stratton, Richard F. 1998. Secrets of the exotic mandarinfish. TFH 3/98.