Fin rot? and what?

I've got a 15gallon planted tank with 2 small angels and a pictus catfish that have been living together happily for several months. This week I noticed that one of my angels' fins (boof is his name) has been changing colour - there is a black/grey fringe. I do a weekly pwc of 25-40%.
Ammonia: 0ppm
Nitrite: 0ppm
Nitrate: 0ppm
PH: 8.3 (stable)
KH: 125
GH: 143
Only one of my angels has tattered fins - I noticed it the week that I got him starting with splitting ventral fins and in my research found out that it is probably fin rot.
It cannot be that my other angel is annoying him since ‘boof’ is the dominate one.
I have done so much research and still cannot find out what it is and therefore what I should do.
Here is a pic of him:
https://2img.net/h/i973.photobucket.com/albums/ae212/nermal47/DSC_7936small_zps54692a63.jpg

(I don’t know gender, but I call boof ‘him’ – all my fish are ‘boys’ until proven otherwise:D )

Looks like normal pigmentation to me. He's a young veiltail with maybe a smokey gene (can't really tell from pic) whch would account for slight fraying and colour of fins. As far as I can tell he's perfectly fine!

I'd agree with Graham, that there's no obvious serious damage, although there are signs of slight damage to the rear edge of the dorsal fin, but that appears to be healing, so personally I'd not be treating at this point, but just monitor the situation.
What's more puzzling is the black pigment, certainly at first glance he appears to be a smokey where the body pattern has temporarily faded out, however, he has an eye stripe which you don't normally see in smokeys. I wonder if the black is new growth where the damaged fins are healing.
I wonder if your water change routine is to blame? Sometimes large water changes can set back the filter bacteria and result in short lived ammonia spikes. Ammonia can burn the fins and result in the sort of damage you're seeing. I would recommend limiting water changes to 10 -20% and if necessary doing them more frequently.

I'm presuming you're using either well aged water, or a chemical declorinator for the water you change, but it might also be interesting to test the ammonia & nitrite levels of any water you use for water changes.

Welcome to the forum by the way!

I usually test the water not long before or after the pwc. Were I live we have really bad water so I have an ro connected to the tap. I've been considering getting a bit of rain water from a friend.
Could it be that he 'grows into' this colour?

He's a happy fish who LOVES his food!

Could it be that he 'grows into' this colour?

Smokey pattern usually develops at about 5 weeks of age, so I doubt it!

Keep a close on on him, and as long as the fins don't deteriorate any further I wouldn't be worried. If they do, and you're sure he's not being attacked, I'd consider treating for skin flukes; but my guess is it won't come to that!

Thanks for all the help!