Fox Face Not Eating

[Uncle99]

I don’t understand, I will add it at the end of this month, do a partial water change.

If you suspect an infection of parasite, then, if the fish can be isolated, then that’s step 1.
Whenever treating fish, there’s a ton of recent information in the related section of this site which can guide you through the process successfully, fallow days very and is not in all cases, the only thing we can do to interrupt the parasites cycle.
I’ve used this section and been very successful by reading posts by @Jay Hemdal, certainly IMM, a great person to listen too when treating sick fish.
I have not been as successful with treating velvet as the attack is fast, death is say 24-48 hours in stark contrast to ick.
But even today, after decades as a reefer, I always research this section to confirm diagnoses, select a treatment option which is both documented to work, and something that I can do.
I believe this, gives the best chances for recovery.