What is this on my Purple Tang??

[disccollector]

Noticed today that my Purple Tang has a whiteish blotches on his bottom fin that is the same pattern on both sides...it actually looks like his bottom fin has turned a faint whiteish color - almost like someone took a paintbrush and brushed a stroke of thin white paint on it.
Has anyone seen this before?
I've had him now for 6 months with no problem. Eating well - Prime reef flake food every 4 days and sushi nori every 3 days.
75 gallon tank (Upgrading soon to 125)
Water(I know these are a bit out of whack..working on it):
Ph - 7.8
Nitrate - 25-50ppm
PO4 - 1ppm
temp - 80
dkh - 20
Ca - 400
sg - 1.024
I have soft corals, crabs, pep and blood red shrimp

 

[beth]

Is the fin frayed? Or does it look deterioted in any way? Is the shape of the blotch irregular? Can you get a pic up of the problem area?
How is the fish acting and how does he look aside from this blotch? Does the blotch look like fungas attached to the fin, or just an irregularity with the fin itself? Is it "pumped" at all?

 

[disccollector]

Hi Beth,
Sorry for the slow response.
The blotch is confined to the inner area of his lower fin and seems to be irregular in shape but it's disjointed square patches that follow the ribs of the fin...it's not rounded or curved so it doesn't appear to be a fungus on top of the fin.
At closer look it looks more that the fin is translucent and not necessarily having a thin white coating.....this would also explain why the blotch is the exact same on both sides of the fin. But the fin is not frayed at all on the edges..it's a nice clean edge...the blotch is contained to the inside center of the fin and doesnot go up onto the body.
I tried to get a pic but they are all blurry b/c he moves as soon as I get close.
He is eating fine and everything *appears* to be normal in his behavior. No labored breathing at all...swimming normally.
So is this breakdown of the fin bacterial or could it be someone picking on him....I do have a yellow tang in the tank with him and they fight from time to time (planning on upgrading size soon) and I have a few emerald crabs and a sally litefoot.
It's been three days now with no help. Would a freshwater dip work?
Thanks so much for responding!!...I couldn't get any bites on the reef board.
David