75 gallon minimum. I kept one in a 50 gallon tank before I knew better. It was fine for a year and a half, then went rouge and quickly killed 3 tank mates before I could catch it. They really get aggressive when stuck into too small a tank.
Interesting. I thought it was pretty much a no go regardless of tank size. I passed up a really nice bicolor the other day because I thought I would have issues with my coral beauty in my 6' 125gal.