Is there a bad odor?
It may be thrush. Here's a little comparison chart that may be helpful. (link below)
Kim (@Eggcessive ) can tell more about contraindications, but Nystatin may work better.
I've never seen it OTC, but an oral thrush paint might be an idea too...
That doesn't sound very good does it.
I'm sorry you're having to deal with all this and starting out with birds that likely will be carriers all their life and may even be weaker health wise long term too.
I don't know if I would treat for canker unless I was certain that's what it is.
I...