Yes, highly possible it's a wound from an attack. From this pic, I don't see inflamed red gills. In fact, they look a little pale so please confirm. It could just be the gill plate is a milky white color. Which fish was the aggressor?
The aggressor was "Chester" a regular koi Angel. It is now only him/her with one redhead koi alone in the tank.
A little Angelfish history time: In the beginning, Koi Angels were made from a number of genetic lines until they came up with a Blushing Gold Marble. That could explain the see through gill covering on your fish. Many other Koi strains have since been developed that don't have the blushing effect so it's not seen on all Kois. So we need to keep all this in mind when diagnosing.
As for the use of the sponge filter in the hospital tank, it depends on the medication but some, mostly antibiotics, retard the beneficial bacteria bed so you want to confirm your med doesn't or it will just kill the sponge filter. When treating with most antibiotics, just use a bare tank and an airstone. The meds have you changing a small percentage of water during treatment so the biological isn't necessary at this point. Just check the package before using any medicines in a tank.
I will have to google image the Blushing gold marble so I better understand.
I did check before the first application of meds and from what I read the Polyguard will not effect the Beneficial bacteria. But I do have extra sponge filters running in other tanks should the need arise.
Place the slate in front of the filter tube to encourage them to use it opposed to the filter tube.
Doh!! why did I not think of that? Hopefully they wait until I get home in the morning...
I still miss some spawns because the fish use the glass when I forget ( or get lazy about) replacing their slates.
