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Hi, my guess would be cottonmouth (columnaris) bacterial infection. Other reasons would be that it is on more than two fish and in Perth anyways, I find summer an odds on chance of fish coming down with it. I can't really tell from picture though.
From memory the multi-cure is MB, MG and acriflavine?
What salt level did you try? Did either of those slow it down or make any difference?
Other question is do you just have access to local fish shop meds? Furan2 anti-bacterial or medicated fish feed would be top of the list to try but these are a bit rare in Australia.
Mainly I've seen triple sulpha as very common or Romet (sulpha & something else which escapes me) in a few stores.
Tetracycline will also be common but is a very old med.
So unless you have any better antibiotics(?) - triple sulpha & tetracycline are going to be the two to try in a QT.