Hi,
With a properly constructed cave, 100 gallon would probably suffice to maintain a small to medium sized China rockfish.
I think you will a pretty heavy duty skimmer as these are voracious fish and need a lot of food.
There are other complications with cold water tanks, and this is probably why they have never really caught on, even though the cold water fauna is very colorful. You should be aware that all surfaces of a cold water tank will have a significant amount of water condensing on them. Public aquaria typically use double-glazed glass to prevent this problem. Additionally, having a tank that is, say, 100 gallons, at 50 deg. F. in a room results in a very significant cooling of that room.
:strooper: