Tail rot?

Right, have an approx 2yr old white axolotl who's tail appears to be falling off!

I know the drill so this is the background info:

He's active, eating and doesn't appear to be affected by it. He gets fed earthworms 2-3times a week, and any un-eaten remains are removed.

He housed in a 25l tank by himself, with black sand substrate, slate/plastic decorations, a filter with spray bar to suppress water flow and an airstone. The tank has been running for about a year with no problems so far.

Water parameters:

Ammonia - 0ppm
Nitrite - 0ppm
Nitrate - 15ppm
Ph - 8.2

The Ph has always been raised but I don't know what caused it to go above 8.0

The only recent event out of the norm was when a corner of his tank got cleaned due to algae build up, it then covered the bottom of the filter, so the filter got cleaned (possibly a bit too enthusiastically).

Heres a photo of his tail, has anyone seen this before or know what it is? And what can I do to help it heal up?

I have an axolotl that had something similar, his other tank mates never developed this. His toes also seemed to disappear too. I tried salt baths, fridging, methylene blue and this had no effect on it at all. It took a while but it eventually recovered and it was initially put down to me using rain water.
http://www.caudata.org/forum/f46-be.../f58-sick-axolotl/57848-damaged-tail-tip.html
I stopped using it and the tail recovered. It did re- occur again a year or so later and I then used almond leaves in the tank, and this cleared it up very quickly.