Thanks for the indepth replies guys
I'll try and get some pictures for you later on. I have actually seen Lotus bite his tail. It gets that look where the rays are in tact but the rest is just shredded. There's never been any sign of any blackness on the edges. It would show up really easily on the white. Peacock is now down to the yellow area of his tail but again, no sign of black.
I have treated the patch on his back with myxazin which would be the right thing to treat the finrot too wouldn't it? He's had 2 courses of that but his back looks that same.
I have another tank exactly the same apart from different lights and a different filter. There's no temp. problem in there, but the filter makes a right racket. The problem is, this tank is right next to my bed so the filter needs to be silent. I can't go out and try every filter out there until I get a quiet one that will keep the temp OK.
The arrangement was heater diagonally across the back with filter intake above the hot end. Now the heater is the other way.
Peacock, who is in that tank atm, actually spends a lot of time at the bottom in the cold area. There are plants enough to rest on nearer the top.
An air curtain is going to be a noise factor again.
It's very difficult to keep a track of whether it's better/worse one day to the next, especially with 2 of them at it. There was def. some regrowth with Lotus though and I thought the tank swap had sorted him, then it was all off again a few days later.
If it is finrot, what to do? As I said, one tank has had 2 courses of myxazin already.