If you just got it yesterday and it has advanced hexamita, I would have taken it back to the LFS and demanded my money back for selling me a sick fish. Unfortunately you can tell by looking at it that it's not well, so they probably would put the blame back on you. Prazi is for flatworms like tapeworm and useless against hexamita, though I suppose it doesn't hurt to do a prazi treatment just to rule out tapeworm. However, the white, stringy feces and lack of an appetite, as well as the wasting away (skinny, sunken look), are usually intestinal flagellates or hexamita. For that I always treat with pure metronidazole powder. I do a seven day treatment, with water changes and redosing every day, as well as keeping the heat elevated to 86F - 88F. I've never tried any of the commercial meds so I can't say how effective they are.
I don't think you need to treat the whole tank if your other fish are eating well and look healthy (not skinny or sunken in), and have black/normal feces. If it's just the one fish you can isolate it into a small QT tank and treat it individually.
