Just by way of update, I lost the desjardini. I think the combination of those couple of days with low oxygen, stress and general lack of appetite were too much. On inspection of the dead desjardini I noticed erosion around the 'temples' (if fish had temples) so it's possible he was being affected by something else also.
The Powder Blue is fine. More than fine.
This kind of brings me to another question. The Blue tang has now been in QT for 3 weeks and shows no signs of illness, in particular no signs of ich. I have not commenced the copper treatment again. AT this point I'm wondering if I should continue to observe him for a few more weeks and treat if necessary rather than just recommencing treatment right now.
I previously asked this as a general question here on R2R and there was a 5 to 1 consensus that tangs should be treated prophylactically. I have previously read articles that strongly argue that this isn't in the best interests of the animal.