Ah well. I was more cut up about losing a powder blue that was doing great and eating like a pig - then just up and croaked. That was a few days ago and now this. My QT tank just went on a meltdown this time. What happened was I had this HOB fuge and the feed pump was choked up with coraline and not causing much flow through it. Detritus settled in teh bottom and festered there and I didn't noticed it. I looked round the back one day and I saw this layer of mist in it near the bottom and when I siphoned it out I almost threw up the stench was so bad. It was slowly polluting the tank and I just can't get rid of green water now. I'm sure that is what killed the PBT and maybe the 6 thought the environment outside looked better than the inside.
Since removed that HOB fuge and I have about 10 days left on this hypo cycle. Remaining fish are a Flame Angel (not looking healthy at all - HLLE or something), a Longnose Hawk (not very active, worried there) and a juvie Emperor who is doing great.
Like I said, disastrous batch but I'll be keeping an eye on water quality next time round.