I'll assume that you have tested
ammonia, nitrites, and nitrates and everything else is going OK.
As well as having finrot, one of my Otos had red streaks under his "chin". I think it was hemorrhaging, which I'm pretty sure just means internal bleeding. I used Jungle Fungus Clear Tank Buddies, which cures that and finrot and other stuff, and anyway now he's fine. The reddness went away after about a week. Note that I also turned up the temp to help with the finrot, and that may have helped with the reddness too. I recommend trying the Fungus Clear, it's cheap and easy to find but also strong and effective.
I hope your fish gets better!