At this point I would not suggest euthanising the fish. HITH can be corrected and reversed in MOST fish. Starting with a good antiseptic is called for. HITH is not caused by bacterial infections or parasites. It's due to many factors.Too many to go into detail here. When a fish gets HITH,usually an infection sets in as well. So many are under the impression that it is caused by bacterial infections or parasites. I would like to repeat that it is not caused by these factors.
Now,what to do about it.That's going to depend on the rest of the inhabitants,is the fish quarantined? If not,it should be to prevent the other fish from causing it stress and killing it.HITH CAN be reversed,not always,but in most cases.
So time for Q and A:
Water parameters.....no saying they're OK or good wont work,I need numbers (before and after onset of HITH would be good)
Inhabatants of same aquarium
size of aquarium
water quality from source (if possible)
meds treating anything in aquarium (everything used up until onset of HITH)
any new additions rock,decor,fish,etc.
water change schedule
There's a hundred other questions,but these will do.
Dozey that is an excellent website for info.
Here's another good site for disease and treatment:
http://badmanstropicalfish.com/fish_...tion.html#HITH