Strange Discoloration On yellow tang

Ok, my yellow tang has been active, healthy, eating regularly, attacking his own reflection, zipping around the tank, grazing off of the live rock, etc.
One thing has me concernerned however. I've noticed on his anal fin there is a patch of discoloration that's kind of orangish-brownish colored. It's as if there is a large stripe on his fin that is this color. i would post pictures but I don't have a camera currently.
My water parameters right now-Nitrites 0, Nitrates 15ish, Ph 8.1, salinity 1.022, amonia 0,
The tang has never had any signs of Ick, and has shown no change in his behavior since this appeared. The fin looks completely intact and there isn't any change in the general texture of his fin besides the darkened splotch
Does any of this sound familiar? Does anybody have an idea of what this could be? Is it an early sighn of some nasty fin disease?

hello? anybody? any ideas?

It's definitely a solid coloration in his fin. It is a thick stripe going from the base of the fin all the way to the tip of the fin in a straight line following the rays in his fin. It is only on his anal fin, and there is nothing else anywhere on him. Like I said before it's a kinda of orangish-brown color. It's not completely Opaqe as you can see light through it like his regular fins. I will try to see if I can borrow a camera so I can show this, but I won't be able to do that for a while.

My Fish is a Freak!! YAY!!