Please help with my sick porc puffer....

[dayta01]

Hello fish fans,
OK, so I have a 55g FOWLR tank that's been set up for about 3 months now. I started off with a few damsels, then added a small volitan, and a few weeks later, i added a small porcupine puffer. Everything was going great up until two days ago. I woke up and found my volitan dead, entangled in my powerhead. This was very sudden because he was eating and acting fine just a day before. Now, my porcupine puffer is looking extremely ill. He is breathing very heavily (like 100 breaths/min), not moving very much (basically he lays still on the sand substrate), and has a sudden loss of appetite. He also looks extremely thin, but I'm not sure if that's because he is sick. I checked for ich on their bodies, but my puffer and damsels don't show any signs of ich at all. I checked my water parameters, and everything is ok (temp - 82, pH - 8.2, Salinity - .021, ammonia - 0, nitrite - 0, nitrate - 5-10). However, just in case, I just did a 20% water change and replaced the carbon filters on my Tetra HOB yesterday. I am completely baffled, and so is my lfs (which I believe to be fairly reliable). I have just set up a quarrantine tank, but I haven't moved the puffer into it yet because I'm a little worried that moving him from being motionless might cause him more stress. Any help (Beth perhaps) would certainly be appreciated.
(Oh, btw, my damsels are perfectly fine so I don't know if its a parasitic infection or what.......has anyone heard of a medciation called metronitazole??? I believe its supposed to be some sort of non-copper-based medication.)

 

[beth]

How did the LFS transfer this puffer from the holding tank into the bag? How did you acclimate and transfer the puffer from the bag into the tank? Puffers should never be removed from water. Stress can cause an inflation and then the release of toxins.
Are the gills swollen and read??? Take a very close look at the gills using a magnifying glass. Also take a look at the whole body. Notice any light dusting on the body? Like powder?

 

[dayta01]

I believe the LFS used a plastic scoop box (I don't know the proper name) to transfer the puffer into the bag, and I did as well to transfer it into my tank. For acclimation, I didn't allow for the 3 week QT process, I just used the drip method in a clean plastic bucket. But as far as I can remember, the puffer has never touched the air.
The gills are red from within, but the gill flaps are still white (not too sure how specific you need me to be), but they don't look extremely swollen. As for the dusting, I don't see a "layer" of dust, but I do notice some small discolorations/ loss of color in a few of his spikes and possibly a slight powder-ish discoloration on his body.
Going back to the acclimation and transfer question, I just wanted to know if it makes a difference that he has been in the tank for about 2 months now, without any problems whatsoever up until this point. Would it still be possible for him to be stressed after such a long transition period?
Thanks for the reply.