bright redness at the base of the pectoral fin where it joins the breast

[xyyz]

hello all,

i just noticed this a few minutes ago. the left pectoral fin's starting point (where it joins the breast) is almost blood red. the entire part joining the body is red, not just a spot. the line isn't too thick, but it's pretty noticeable.

the fish is still active and has a good appetite. there's a pair of aggressive angelfish that are close to spawning, so i know they take nips at all other fish, making me think this might be a wound; however in looking at the pattern, it seems unlikely.

is this a wound, or is this, and i hope not, some sort of infection.

what do i do to treat this condition?

i have a quarantine tank ready, so if the fish needs to be treated, i can get the meds and start on it right away.