Angel Diagnosis help so I can treat (or not)

[Janetsplanet]

I picked up a small watanabe angel, and at the time I didn't notice she had a very small white mark between her eyes. Luckily, I took photos before releasing her into the aquarium, and further inspection indicated she had this from the start.

I'm relatively sure it's not a scratch because over the weekend the sore has increased in size, and looks a little ragged (maybe peeling?). Based on the pics of HLLE I found online, I'm unclear what this could be because HLLE doesn't look "white", but more like skin color or dark pockmarks, and it doesn't look "ragged" to me.

She's being a bit shy, but is eating like a piggy 2x a day (mysis and rods food mix). I offered a tiny bit of nori, but she wanted nothing to do with it.

Best and most recent pic attached. Thoughts, ideas suggestions?

She's in a "QT" (sort of) - Old aquarium, with sand and live rock, minimal filtration otherwise. (54 gallon + sump)
Temp: 78
Salinity: 34 ppt
Nitrate/Phos: 0
PH: 8.3-8.5
Alk: 7.5
Cal: 420
Mg: 1350

 

[Deinonych]

Difficult to determine from the photo. Can you take a picture without the actinics on (or use a flash)?

 

[Janetsplanet]

Took loads of patience, but snapped a couple that may help.

 

[Janetsplanet]

LFS gave me some Seachem KanaPlex to try feeding her (still eats like a piggy). 24 hrs no change; in fact it seems worse tonight. Am I on the right track, or should I try something else?

 

[ThRoewer]

It's likely bacterial. It's not likely to see instant change with antibiotic treatment, though if you don't see improvement within 2 to 3 days I would change to another antibiotic.
Kanamycin treats infections caused by Gram-negative bacteria, so if it doesn't work I would try a broadband antibiotic that is also effective against Gram-positive bacteria.

 

[Janetsplanet]

Sad, this morning she refused to eat her medicated food and ultimately didn't make it...

Now a follow up question... The QT is 65 gallons total volume. Do I need a fallow period, water change, treatment (all of the above?) before I use it for any new additions?

 

[ThRoewer]

I would break it down, soak everything in bleach for a few days and then let it dry for at least a week before even thinking about setting it up again.

Since you don't know what it is I would be super careful.

One of the possible things that may have caused this would include Uronema marinum - no fallow period will eliminate that one!