Fish Scales2;629079 wrote: Miami, great idea and it often works well.
Multiple tangs will take a system much larger than 90 gal. Especially when they all have the exact same shape. They will (if under 3") survive for a year or so but eventually the tangs will battle for territory and the smaller weaker one will succumb. (most likely the Scopas, kinda sounds like whats happening now) If these tangs are over 3" than it may be time to rehome either the Yellow or Purple. (I would go with the Yellow as the Purple is your favorite)
I am not trying to criticize your choices (in fact I adore each of those fish and have them in several tanks that I maintain) just expressing what I have learned through my experiences. If I were the tang police I would have been SHOT years ago. I had close to 20 tangs in my 300 at one point.
One other thing you can try is pulling the yellow and purple for a week and drop into a QT tank or sump. Allowing the Scopas to strengthen and heal, both physically and emotionally.lol He will gain a sense of confidence IMO and become bolder. When you add the larger tangs back in they will most likely be much more chilled. For a while at least.