I am new to the forums, but I have been searching obsessively to find the answer to this same question. My fish have this SAME disease I have been wracking my brain to figure out what it is.
Sorry to hear that Pete.
At this point, my plan of attack is to continue with the Parasite Guard tablets, then try the Leva and Flub from Charlie, and then move on to the Kana (since it's rumored to help with TB). I'll keep you posted on what happens, but results are going to be slow in coming.
Since we're both paranoid that it's TB and I've already got the Leva and Flub on the way, would it be worthwhile for you to start with the Kana instead, as a sort of multi-vectored attack?
Let us know what happens.