Hwg, I was thinking about this today again and if it is a parasite, the antibiotics might not do anything. I would try the aquarium salt (don't use table salt of course). I was also thinking that, unfortunately, sometimes fish develop various tumours, etc. which don't really have a cure. I hope that's not what it is...

Hey G! I added some aquarium salt last night. I basically decided to finish up this second round of antibiotics (treatments today and Friday), and if there's no change I'll try an anti-parasitic medicine (like Maracide or MarOxy).