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.