There are several factors that are essential elements for increasing the odds of success with mandarins (BTW, I have kept numerous pairs for many years at a time).
1. The age or maturity of the set-up. It is advantagoues to wait one year to allow the microfauna and small creatures that will be the stable of thier diet to grow into large numbers before a mandarin or any fish that competes for the same food (i.e sixline wrasse, etc.) is introduced to the tank.
2. A Sufficient volume of live rock
3. A deep sand bed of mixed sized grains to support a diversity of live foods
4. Compatible tankmates that do not compete for the same foods. Also do not keep sand sifting fish such as a dragon goby which will consume some of the live foods (i.e. adult, reproducing copepods) living in the sand
5. A healthy refugium
6. A tank big enough to support a large enough population of live foods in the aquarium
7. Feeding supplemental foods enriched with vitamins and highly unsaturated fatty acids. Most mandarins will accept some supplemental feeding. Training them to these foods should be accomplished during an isolation period in an smaller aquarium before being placed into the display. This can take patience and persistence. Do not attempt to make supplemental feedings the staple of their diet. Mandarins do require a substantial amount of live foods to be constantly available and growing in the system. I do recommend practicing quarantine with newly acquired specimens. I do this by placing them into a separate tank that I use as a food farm. It contains live sand and live rock and a variety of foods including worms, amphipods, mysid shrimp and of course copepods.
8. Making a wise choice when selecting a specimen. A large percentage of mandarins that I have see offered for sale are in some stage of emaciation. Most of them are not fortunate enough to end up in a LFS that knows how to care for them. Look underneath them to see if their stomach area is concave. Medium size (not too tiny or large) specimens are probably the best bet.
9. Some sort of cover on the tank, because mandarins do sometimes jump during the courtship ritual or when frightened (usually at night).
10. You may have success without following one or more of the elements I have suggested, but if you do follow them all you will have a greater chance of long-term success.
Terry B
BTW, I don't monitor this message board often so if someone has a question they want to ask me the best bet is email