Not especially difficult to keep, but do go over the threads about them in the *brackish* section because -- despite their name -- the species usually traded are brackish water fishes. Basic care is identical to marine moray eels, so any information you read about them in marine books holds true here. They're potentially large, certainly predatory, and only unreliably tolerant of tankmates. Good escape artists. To be honest, the marine morays like the snowflake moray are perhaps more fun (and certainly prettier). Marine morays are very hardy animals and not in the least difficult to keep.
Cheers, Neale