For some reason seahorses & pipefish are less prone to parasites. I used to think it was because they were scaleless, but that quality doesn't seem to help puffers with Ich/velvet.
I actually was talking to a PA this morning and asked how they QT their ponies. Apparently they tolerate Copper(II) sulfate pentahydrate quite well

:
https://www.reef2reef.com/threads/diy-copper-med.364112/
The guy I talked to also said they are more likely to find Ich/velvet trophonts inside their gills than on the skin.