Garlic helps with stimulating appetite for picky eaters. It is argued that it boosts the fishes immune system, but that seems more a belief than actual proven science. I kinda think its that the fish that is eating is healthier than the one that's not. Garlic is absolutely not a treatment or cure.
For treatment, its tricky without a QT. You could do the freshwater dip. That supposedly gets a lot off the fish, almost instantly. I'm not sure about Formalin in a DT. I've never used it. Apparently if the fish is cured, but the DT is not treated, this parasite lives on and can come back.
, getting a 10-20g QT costs a lot less in the long run than losing fish. $10-$20 for a tank, $30-40 for a power head or small filter to keep water moving, a couple pieces of PVC
from Home Depot and a heater. Probably less than $100 if you go high end prices. If you add fish later without QTing first, they could have Ich, or something like Marine Velvet and you could lose all your fish.