Spironucleus vortens, aka hexamita cannot be cured with garlic, in Discus, or anything else. Garlic has it's place in preventative measures, but is certainly not a cure for a fish that is heavily infested with internal protozoa.
Praziquental is an anthelmintic cestodicide, designed to treat flatworms. In other words, it's best used as a deworming agent, not to treat internal flagellates such as spironucleus/hexamita.
http://www.int-res.com/articles/dao/45/d045p197.pdf
ABSTRACT: Hole-in-the-head disease is recorded in 11 discus Symphysodon discus Heckel, 1840 and 1 angelfish Pterophyllum scalare Lichtenstein, 1823 obtained from local aquarists within the Southwest of the UK. Spironucleus vortens Poynton et al. 1995, was isolated from the kidney, liver, spleen and head lesions of discus showing severe signs of the disease and from the intestines of all fish. The hexamitid was also recorded from the head lesions of the angelfish. The identity of these flagellates was confirmed as S. vortens on the basis of topographical features seen with the aid of SEM. A modified in vitro culture method was successfully developed for the detection, isolation and long-term maintenance of S. vortens. The flagellate was sub-cultured at 3 to 5 d intervals, new media
being supplemented with fresh liver from Oreochromis niloticus (Linnaeus, 1757) free from infection. The results are discussed in relation to S. vortens as the causative agent for hole-in-the-head disease following systemic infection via the digestive tract.
And this is how to best treat spironucleus.
http://www.monsterfishkeepers.com/forums/showthread.php?339362-Treating-Hexamita-aka-Spironucleus
While Metronidazole can also be an effective treatment, it's also hard on fish if used over a prolonged period, and as stated in the spiro cure link it has been shown that in fish, an excessive use of metronidazole can damage kidneys and other internal organs.(Bassleer, 1983)
Most experts in this field suggest no more than 3 treatments of metro via the water, and/or 5 days via feed, but if via food it has to be carefully measured according to the fishes body weight.
Last but not least - why subject your fish to meds of any type, without even knowing if there actually is an issue? That makes about as much sense as giving your kid antibiotics when they have a mild cough.
Magnesium sulfate (common epsom salt) is very safe for fish, with none of the potential drawbacks of standard medication such as metronidazole. Worst case scenario with the 3% epsom salt solution is your fish has to flush some excess magnesium out of its system for a few days, best case scenario is the treatment cleans it out of some internal protozoa that was causing it some mild internal issues.
It's a win-win for you & your fish.