Large white lump on comet - a tumour?

[NB55]

Hi Everyone, I am hoping for some ideas/comments on my large comet who has what looks like a massive tumour on the rear of his anal fin close to his body.

Rhonda (yes I know I found out too late she was a he but by then the name had stuck - poor guy, he is the lucky one, the other large male is called Wendy, but I digress.....:-? ) Rhonda developed the start of a white lump about 2 years ago, it has slowly grown through the fin to the size of a marble - monitoring him he seems a very happy fish, fin health otherwise good, eats like a horse, swims happily around the tank and loves racing with his mate. Reading up on it and talking to a pet shop owner who specialises in fish I decided to leave him to keep on swimming rather than medicate like mad - let nature take its course as he seems so happy. A few months ago part of the lump turned black and split open slightly - the lump heals and then splits again every few weeks - unsure if this is going to be a problem. I would rather not separate the fish - for starters I only have one tank large enough to comfortably house 2 x 15cm long comets who like to spread their fins - also assume any transmittance of disease would have infected Wendy by now - I could imagine the floaty-split skin could be quite tempting to nibble on although have not seen this ......

So has anyone else out there had experience with slow growing white lumps that split - I am guessing it could be one of a number of things and while I've had fish for a few years I would say I am a total beginner when it comes to diseases. I have been dosing with Melafix and this seems to help the lump heal up but not decrease in size or stop the black (necrotising?) part from appearing. Also have a small amount of salt in the tank. Any other ideas I could try to make him as comfortable as possible? (Possibly more uncomfortable for me looking at it, he is a perky as the day I got him ) Any comments gratefully received, in case I am missing anything obvious.

Sorry no photos so show, would have to get hold of a camera. Cheers, NB