Clownfish fin potential issue

[fishkeeprian]

Hi I have one clown in QT and the fin next to the dorsal fin is black with an almost translucent edge around the black and that is becoming frayed, not the black part.

Any ideas?

https://www.dropbox.com/s/3glotc63a15v20f/Screenshot_2017-11-10-22-59-12.png?dl=0

Thanks