Oh no!! I'm so sorry the damsel is dying too. I don't know what you can do about the PVC
It seems weird that the hermits are doing fine. Here's what happened to me with freshwater-
I got a tank off of Craigslist. Cleaned everything put stuff in it, filled it, made sure it was cycled. Time to add fish! Feeder goldfish- added 5, they all died. Next I added some other fish- rummy nose tetras- 7. 5 died. Neon tetras - 5. I think all of them died. Anyway, my husband called it the tank of death. I think the LFS
was going to stop selling me fish. Added more fish, tetras or platys... I don't remember. Finally the fish stopped dying. I have no idea what the problem was. I lowered my pH. That's the only thing I could think of.
Saltwater- live rock and all that jazz, tank cycled. Added hermits, snails, emerald crab. All fine! Clown goby- died. Clown goby 2- died. Clown goby 3- died. I blame poor stock. All my levels are fine. Corals are fine. I've been fishless since October or so. I needed to make sure my system was free of Ich. The last clown goby lasted a week but then developed Ich. Ugh.
Either your water is poisoned from the PVC
cement or your LFS
has very poor quality stock. That's all I can think of. I hope this problem gets solved quickly!