Betta Tail Biting Or Finrot?

Hi everyone,
Been a while since I posted anything, but I still pop into read posts nearly every day.
I don't know if my Betta has fin rot or if he is biting his tail. I've looked at photos on the internet, and I still don't know
I have tried to take some photos, but he is constantly moving and what I have got are pretty poor. I thought I'd post them anyway.
I have also posted photos of what he looked like when I first got him and he has obviously changed colour quite a bit........as you can see, his fins have gone from white to black and he has lost the white cap.
He is very active and is eating well.
The water is good , with no ammonia, nitrite and nitrate under 5 ppm.
I have some Esha 2000, but I am loath to use it if he is not sick.
I am at a loss as to what to do, to be honest.
Any help would be much appreciated.

[img][https://imgur.com/gallery/DDDPA/img]

I've forgotten how to link to photos! Will try again

The colour change from when he was younger is because he has marble genes. I had a betta bought as a blue and white marble which turned plain blue within weeks.

As for his tail, it's hard to say. Do the edges have any redness to them, or black or white edges to the damaged areas? You say he is active and eating well, which would indicate he doesn't have any serious infection.
The problem with tail biters is that they don't do it when anyone is watching, they pay attention to the human instead. If you could spare an hour of sitting motionless as far from the tank as possible, you might catch him biting his tail, but that's probably the only way. And I mean absolutely motionless because if he sees movement he'll stop.
It is usually bored bettas that bite their tails, but your photos show a tank full of things which should stop him getting bored.

But what to do. In the absence of any red streaking, or black/white edges I'd be inclined to try daily water changes and see if the helps. But if there are any of those signs, I'd use the esha 2000, followed by more often than usual water changes while the damaged areas heal..

Maybe just do a low dose especially as only fish in the tank...

Another "halfway house" option might be to try something like API Melafix or Waterlife Nova +. I've used the former when in a quandary about whether to medicate or not. I've never tried the latter. It would be worth checking if they can be used with bettas, though.

Update

I have been doing daily water changes for about 10 days now and I think his tail has improved.

I've also resorted to putting a mirror close to the tank, only for a minute or two about 3 times a week, in the hope that it would relieve any boredom that may have caused him to bite his tail.

He is still active and still eating well, thank goodness.

I was wondering when I could go back to a normal weekly water change?

I must say, that I am still mystified as to what went wrong with him. I would have thought that if he had tail rot that he would not be so active, or eating so heartily.

I don't expect I'll ever really know the cause, I'm just pleased that he seems to be on the mend

Thanks very much Sue, for your help and advice, it is much appreciated.

I have noticed marquismirage, that since I have been doing this, he patrols the tank a lot more , as if he's on the lookout for the "interloper". 

And yes, he does look very impressive/fearsome, when the gills and fins are flared.