Tangs not meant to be or what???

[DSlater]

I know a lot of people don't like UV's but just throwing it out there.

We had some bad ich outbreaks about a year back, our powder blue was just covered with it. Not being reasonable to set up quarantines for all of our fish, I found a good UV system. Took about 6 weeks till there was no visible signs of ich, ran the UV for another 6 weeks to be safe and took it out.
The UV wasn't cheap but cost less that replacing our fish.

Sorry to hear about he loss of your tangs. Have you thought about being patient and waiting for a good healthy one to outgrow another tank locally? Two of our tangs came from other tanks, already a good size and healthy.