Odd Patches On Clown Loach

Been a while since I have been here - apologies that my first post back is a question! And a long explanation at that. Bare with me...

My husband and I have kept clown loaches for several years. We have 8 in a 240l tank (planning on upgrading it this year before they outgrow it). 6 of them we have had since they were very small but two we got later. Those two were returned to our LFS after outgrowing someone's tank. The LFS couldn't tell us much aside from that they had been quarantined with no issues, and that they were probably stunted (our LFS is good - independent and trustworthy).

We think the two newer ones are about the same age as the others, so all our clowns are approx 5-6yrs old. We got the newer ones about 2 years ago.

As one of them has grown bigger however, we have noticed patches where the pattern to her (I think) body is discoloured - almost as though the black stripes are torn off leaving the pale orange from underneath. We first started noticing this about a year ago. The fish was pale and looked different when we got her, but we put this down to stress and an abnormal pattern. The other one we got was and still is fine.

The patches seem to be getting worse, but we can't identify the cause. None of the other fish are affected so it can't be a parasite or infectious disease. She is also eating and interacting normally and appears to be the dominant loach. I check the ammonia/nitrate etc weekly and all is normal. I do approx 20%water changes weekly.

Other tank mates are blue/three spot gouramis, harlequins and lemon tetras.

We have tried dosing with a general tonic medication, didn't have any effect. It also is absolutely not white spot - the patches are too large, there is no flicking and the others haven't caught it.

Only thing I can think is maybe she has ammonia burn scars from the previous tank and it is looks worse as she grows bigger? Or could it be a hereditary disease - can fish get vitiligo?!

Anyway I don't want to try medicating again and adding stuff to the tank unnecessarily. She seems happy enough and it isn't affecting the others so I am tempted to leave it be and assume it is a quirk. But before doing this I would be grateful for any input from others just in case.

Pics attached. Sorry for the reflections, I am rubbish at photos and she is pretty fast so this is the best I could do.

Hi, I'm not sure if it is the same thing but I have a male panda cory who's got what sounds like similar patches on his body. He's also got a funny side fin - it looks like he's got some kind of a wart on it. Like you, I tried to treat it thinking he's got some kind of external problem. He's been through anti-biotics and melafix but he still looks the same. He's been like this for nearly a year now. He's never passed it to any of my other cories and he's the father of at least one of my young panda cories that was born in the tank and he's not passed it that way either.
I did try researching what was wrong with him but couldn't find anything. The only thing I did find was a possible answer to his funny fin - that could be caused by excess protein but as he eats the same as all my other cories (and I have over 20 cories) and none of the others have this so I ruled that out.
 
What I would say is if the clown loach seems healthy in all other ways, is eating fine and behaving normally, I'd monitor it but do nothing. If it isn't spreading it's likely what you've already thought and something he's been born with. I suspect my cory is the same. He's got some kind of problem that he was born with. 

Do the patches look on top of the skin or beneath the skin?
Any signs of rotting flesh, or dead skin around the edges of the patches?
Any fluffy cotton wool on the patches?
Any signs of flicking and rubbing?
Do the patches ever disappear and reappear again?