Lots of research! (Sorry)
Normally it is quicker to administer a treatment, learn as you go.
Internal parasites are dealt with by a few off the shelf type treatments, they are normally broad ranging and knock out a few types of pathogen.
You'll get a general parasite treatment an internal bacteria treatment and a worms specific for internal disease, external ailments similar.
Looking at the picture (post #10) I would guess cavity worms, however, I had a Cory up to and over Christmas who had a similar lopsided lump, I tried everything I could find and I ended up "doing the right thing" in the early part of this year. (She was old, could have been a tumour? I didn't dissect her)
Normally new fish I treat as a matter of course for everything then I know what it isn't.
If you use meds, read the instructions closely and follow to the letter. I use water life sterazin for worms and API internal parasites/bacteria for most if not all of my internal issues.