exactly.....minimum for one..
In my experience, most sites/stores/books that talk about minimum tank sizes usually have them grossly underestimated, probably because they know that most people will be put off if they read about fish that really require hundreds of gallons to properly care for. These sources are in the business of making money, and will tell you what you want to hear so that you will spend, spend, spend.
IMHO, a WELL-ESTABLISHED 100 gallon tank would be ok for something like an emperor angel to start, but in order for the fish to remain healthy into adulthood, you will eventually need a tank twice that size.
and your not going to get them to grow to 15" anyway in captivity. It's even rare to buy them that size from the wild.
I beg to differ. I've seen emperors close to 15" for sale at my LFS.