I agree with Anemoneman. I had a beautiful longnose butterfly fish, it was eating very good and swimming all over the place. Beautiful color and then one day, it just lost all the color and died. This process took about 2 hours. My water parameters were near perfect.
For the life of me I can not figure what happened to it. I had it for a couple of months with no problems whatsoever with it or with it's tankmates. Everyone got along just fine and still do. Kinda like the same thing that happened to the Rev with his Moorish Idol.
I would suggest that you really think about this one. I don't know about you, but it breaks my heart when I lose a fish, they are like my kids. I try very hard to make them a great place to live and take the time to keep the water as close to perfect as I can possibly get it. It really sucks when you put so much time and effort into something and one of your little friends dies on you.
But to answer your question, all my books say that you should have a minimum tank size of 75 gallons. Can be housed in a reef tank but with some caution because they can (but not in most cases) feed on corals.
125 Gal-35 Gal Wet/Dry-Tetra-Tec PF500 HOB
(3) AuquaClear 301 Powerheads-600 GPH
Automatic Tempature Controller
5-6 Inch Sand Bed-200+ lbs of LR