My friend, unless you are an advanced aquarist with delicate species/invert experience, I am sorry you made both of those purchases. I say this not to bring you discouragement, only to educate others. Here are excerpts from my post on another board for both species:
Mandarin Dragonets (Synchiropus picturatus, splendidus)
Sometimes called Psychedelics, gobies, scooters etc., they are not gobies, but in fact a beautiful and fascinating species that most often starve to death due to slow behavior and demand for continuous live foods. This is not an impossible species, (some have good success providing the right care and non-competitive tankmates) just specialized beyond the realm of most hobbyists. Established pod colony or not, you would likely never purchase another one if you could see the millions which have died while in better care than you can provide. I have seen these wild in the lagoons in Micronesia; they are stunning in nature and deserve better than to slowly starve in our glass box... not for beginners or most aquarists for that matter.
Blue Starfish (Linckia)
Over 90 percent mortality in captive systems, usually due to shipping damage.
Not for beginners. While diving in Palau, some of my most vivid memories were formed from these dazzling echidnoderms in shallow water, sunlight dappling on the brilliant blue... makes you appreciate these in a different way, and encourage beginners to avoid them.