True perc clown losing black

[Heidemann]

Quick question for everyone.

Several months ago I purchased two true perc clowns. They have been living peacefully together for about three months in their host anemone without issue. When I got them they were both identical is size, shape and color. Over the months one had grown much larger so I am guessing that one is turning into the female. The issue is the other one lost almost all its black color. The orange and white isn't faded and it eats and swims without issue so I don't think he is sick. Doing a quick google search I couldn't find any information about this.

So what do you think?

Here is a picture from a while ago...

 

[jski711]

I have heard of onyx percs losing color for no apparent reason. A local breeder around chicago had this happen to one of his pairs and has no clue why.

 

[Woodstock]

I've read about that happening to fish but no one can determine the cause.
He is still pretty

 

[TheOtherReefer]

I have heard that it might have to do with their host anemone. I have heard that BTA can be a cause. All speculation but read it in multiple places.