Not much to add. As Dave says, one of the two key things with these fish is oxygen. They are totally intolerant of low oxygen levels and will come up to the surface and gasp when so stressed. The other key thing is feeding. They are nocturnal animals, and while they happily take frozen bloodworms, they can't compete with, say, catfish. So you need to make sure that they get a chance to feed properly.
Beyond that, these fish are not difficult, and well worth trying. A kept a coldwater plaice in a room-temperature tank when at university, and it got very tame. Even ate trout pellets!
Cheers,
Neale