I’m beginning to think the lfs providing the information is leading the op down a rabbit hole of failure .With chemiClean which I despise, filter needed to run. Skimmer can be turned off and should. With filter off, oxygen reduced. At minimum if turning off a filter, add an air stone for movement and oxygen exchange
The tail looks shredded suggesting aggression from a tankmate (something a clown would do)
So much information that might have been the way 30 years ago .
but so much has changed . Such as the old saying of using old sand helped skip cycle and provided good bacteria “
. I do not believe vibrio is present in most marine aquaria (but IMHO one should act as if it is present). My other point was that since erythromycin can KILL vibrio vulnificans according literature (depending on the location found) - its not likely that chemiclean for 2 days will cause resistant organisms - which tends to occur when antibiotics are used for longer periods of time. But - again you're absolutely correct IMO - avoiding unnessary use of antibiotics - especially in a display tank - as compared to a QT tank should be avoided if possible.

