Is he a halfmoon?
I know some half moons have this condition where their tail will start to dissolve in hard water. :S It's because their so inbred that they have that problem; their fins aren't as strong.
Aros had it happen to him for a while, I was freaking out because I thought it was fin rot! It slowly ate away at his tail until he practically looked like a plakat, but the edge of his tail looked similar to those pictures. I had to use black water extract in his water and I would have to test it to make sure the water was soft before I put him back into it.
But the middle section of his tail looks like he's been biting it. :S
[edit:] oops should have read the second page xD;;