Your photogrphic evidence mean nothing to people how keep these  trouble  some Tangs without UV's for many years.Besides 3 years isn't even a long  time to keep a  
Hippo Tang   alive.Nice job though, i hope you can keep the Powder Blue for much  longer.Regaurding the use of a UV.Yes they work for me.Over the years of  observations i came up with my own method and specifications i use 30  watt UV's on my fish only tanks.Reef Tanks i may add, the corals didn't  do so well. 
Every tank is different, even if you use an over size  UV it doesn't necessarily mean it will do wonders.There is much more to  it but as i said they work for me.