Actually I've thought of something more specific
What is the difference between Praziquantel and Metronidazole? It seems the first is more for worms(?) and the second more for protozoal parasites(?) but I don't really understand what these do or what sort of chemicals/drugs they are?
Well, you have probably read quite a few of my goldfish posts! Standard ich protocol for my guys is .3% salinity (raised gradually over 48hrs then reduced gradually after treatment) and maintained for minimum of one week after the last spot is gone. Large, frequent wcs are part of this. On a very rare occasion, I have gone as high as .5% salinity but this is rarely ever necessary and I would prefer to consider other alternatives if possible. 99% of the time, treatment takes 10-12 days tops and it always a fish that is already in quarantine. Please ask if you need more specifics!
Prazi is 'technically' a de-wormer but it only addresses very specific worms (tapeworms and some trematodes). It's best known as a de-wormer in canines and felines as tapeworms are a common issue in household pets. Although fish can get tapeworms, it's been my experience that this is not a big issue. Perhaps it's a different story with wild caught fish but I can not speak from experience so I honestly do not have an answer.
Prazi's big benefit in fish is in treating flukes. Prior fluke treatment options were very toxic to fish (and every other aquatic creature) and difficult to remove from an environment once administered. I have used prazi on fish as young as two weeks, in planted tanks and with inverts without incident and it's the one medicine I always have on hand.
Metro is another medicine that I also like to keep stocked. It's mostly commonly known as an antibiotic against anaerobic bacteria. It's also effective against some Protozoa and parasites. It combines well with prazi (such as in API General Cure) to address a more broad range of issues. It can be very effective in treating a fish with symptoms of an intestinal issue (weight loss, bloating, lack of appetite, odd waste, etc) and is a frequent choice for fish that is just starting to dropsy.
Of course, these are very broad generalizations here and not intended to specifically diagnose or treat any creature. If you more specifics, please ask!