Thanks for your help!
Just added a bunch of catappa leaves, see if it improves. I'm in the UK so getting my hands on any fish antibiotics is very difficult unfortunately. I could possibly order some of the seachem stuff, but it would still come from abroad after a week or two, at a very steep price. Which one of those is worth trying?
I also have some human pills left over in my medicine cabinet (amoxicilin and moxifloxacin). Not against trying these, but not really sure if they'd just kill the fish outright, and what kind of dosage to use.