there is no one cure all.
qt is a process to start the QT tank itself (imo) should be un-medicated, low light, quiet place for a fish to relax and de-stress after its long often rough journey, with lots of hiding places (pvc tubes) and ultra clean water. a place that it doesnt need to compete for food. a place to regain strength.
The fish should be observed for signs of problems and then treated accordingly, with dips(baths). Prazi is great for flukes and worms , fresh water or a hypo salinity for fluke and ich.
Some fish should be dipped regardless in certain dips. its a learning thing, for instance sharks that where not born in captivity should always be dipped in Prazi for worms.
Some fish get secondary infections from the travel or parasites , and need antibiotics.
even when people travel to far off places in planes or on boats we get sick from the confined space .. cramped conditions ect.. Treating those who get sick is done on a one by one basis. The QT is just a place to keep the fish so your DT is safe, not a place to treat.
Good luck I am happy to offer any specifics if you need anything.