Not sounding to good, if you can remove the plec as they can't tolerate salt, but neons can tolerate small amounts, do a water change add the binox back to the water removed and add the maracyn with it as well.
Forget the melafix don't add anymore.
Increase aeration.
Maracyn
Manufacturer: Mardel
A broad-spectrum antibiotic for gram-positive bacterial infections. For infections of columnaris (body fungus), fin and tail rot, popeye, gill disease, and secondary infections.
Active ingredient: Erythromycin.
Keep an eye on the fish as i don't really like telling fish keepers to mix meds, but with columnaris you have to use maracyn one and two, so i would carry on with the binox with the maracyn one, good luck, fingers crossed.
But please make sure the tank has alot of aeration.