Comet with red, 'threaded' fins

Hi folks, I was wondering if anyone on the group could help me work out
whats happened to my comet.

Hes about 8 years old and is about 12 - 13cm long. He's currently in a 13
gallon tank (bit small I know, but finances don't permit an upgrade ATM).

I noticed this morning that he was lurking at the bottom of the tank
behind ornaments etc, and upon further examination I found that his fins
and tail were a very dark red, and had 'string' like things hanging off
the end of them. In fact, a significant length of the end of his tail was
missing completely.
I investigated further, and found that these 'strings' were in fact the
veins of the fin (don't know their scientific name sorry), it appears
that the 'webbing' of the fins have been eaten away by something, leaving
just these veins trailing.
The severe redness of the fins and tail leads me to believe it has been
burned in some way... possibly by the water PH or similar.

What I wanted to ask was, is my analysis of the problem accurate, or is
there a disease which can cause the above ?

There were 3 other fish in the tank, a black moor and 2 fantails, all
relatively tiny. These were removed 5 days ago because the black moor was
constantly bullying the comet and attacking his tail.
The thing is, he did not have this damage when the other fish were moved
away, so I don't think that was the cause.

He is currently in a bucket and I have drained the tank completely to
wash out the gravel and filter...(I found a piece of the missing tail end
while I was doing this :-\ )
I guess my question now is whats the probable cause of this, and what can
I do to try and ensure his survival / recovery ?
He is still swimming and feeding, which is a good sign at least.

Any help or advice would be greatly appreciated,

Thanks.

Ciarán.