Chainlinks are secretive species that spend most of the day hiding under or between decorations/liverock, so no worries - that's normal. Plenty of hiding spaces is important for these eels.
In the wild, they primarily eat crabs and occasionally shrimps. If it doesn't eat, try putting in some live fiddler crabs or grass shrimps in your tank, as they may need live feeding to kickstart food intake. Then you can try pieces of shrimp on a feeding stick or a tweezer.
As a minimum you want 30 gallons per animal and keep it between 72 to 80 to keep it alive.
If you want to keep with other fish, they may snap at them during feeding time, but seeing as they don't eat fish in the wild you have room to play around with.
As a general rule, morays are hardy as their slimy toxic bodyslime repels away external parasites. If infected, they will flick their dorsal fins up and down, head-shake, or rub its head on substrate.
Internal parasites are rare, but they happen. Symptoms are decreased appetite, weight loss, increased appetite with weight loss, heavy breathing and loss of colour. If infected, treat them with Fenbendazole, Piperazine or Metronidazole.
If the water quality goes bad, it may get fungal/bacterial infections.
Avoid fiberglass decorations as they may get skin lensions.
Morays are sensitive to copper and organophosphates so make sure of this when you are treating the tank.
Two excellent books on moray eels:
1. keeping moray eels in aquariums by Phil Purser (cheap)
2. reef fishes vol. 1 by Scott W. Michael (not-so-cheap)
