Copperband gill tumor?

[asylumdown]

What do you guys think? I just noticed this today, it's large enough that it's preventing one gill flap from closing completely. Fish is behaving normally and still eating aggressively.

This guy has/had lymphocystis. He had some really bad fin growths when I got him, but they went away within days of leaving quarantine and he's not had a recurrence since. I've had him for well over a year.

Everyone in the tank has been proactively QT'd with the tank transfer method and treated with prazipro. I added a mandarin goby a couple months ago that never ate and starved to death, but other than that I've not added any fish since before Christmas.

Other than highish nitrates (around 5ppm), nothing about my parameters gives me pause.