Three days with nothing eaten is getting to my edge of pulling from DT.
My Male doesn't eat much in compare to the female, in my case, this coincides with pre-breeding, but he does eat a little each time and remains strong.
The number a days not eating combined with an eat and spit does cause me concern as it's clearly not normal. No change in one more day, I would isolate him in QT, there we can control his environment.
Theirs no question in my mind that both Ick, as well as velvet, can cause fish to stop eating as both can infect the gills before showing up on the flesh. But, they are not the only reason for sure.
So.
Do not use copper unless you KNOW you have Ick or velvet, and if used, only in isolation and in amounts stated by the manufacture.
You can use hypo for Ick, but not velvet.
Forget dosing the DT with anything, for now, no changes to DT.
Never use DT water, virus, bacterial infections, parasites are in this water, keep in mind the Q is for quarantine.
Forget the PH
Glass, water salinity, temp same as DT, (but I lower my QT salinity to 1.019-1.020 as this salinity is much easier for the fish to process, so less stress, something to hide in is important, keep lighting low.
You mush watch ammonia, because the QT is not cycled,but if you do a 1O% WC daily and vacuums up poop and uneaten food, no reason to cycle.
To me Prazi is a " general " cure, a broad spectrum, good for bacterial infections, useless on parasites, but, again, you have yet to see a parasite.
If he does have, they will multiply and you will see them along with lathargic behaviour, rapid breathing, glassy eyes, white dots.
Based on what I read, I would try only one more day, if not eating, pull him as he will weaken and this is where parasites take hold. Keep him in new water 1.019-1.020 salinity, normal temp, OK for "general cure". Keep in mind that a side effect these broad spectrum cures is suppressed eating but if he's not eating to begin with, I certainly would employ this
There's a member call Humblefish on this site which just wrote (fenbendazole) on intestinal type medications, it may be of help to read and will be interesting if he chimes in here
I hope this gives you some choices.