I agree with sk8r, it will probably heal just fine if it was just the fin that was damaged. I got a trio of bartlett anthias and they got lymphocystis on their fins when I first got them and into QT. I had to carefully remove each one, lay them on a damp towel, and remove it gently with a razor. They all had pretty bad nippy fin afterward, but it grew back no problem.
On a side note, I think petco has the $1 per gallon sale on glass tanks, so you could get a fairly cheap 10 gallon tank if you need an extra to let him heal.