If it's on the open market, I would be more than cautious with "Parazap". First, I would really be surprised if any drugs could cure both internal and external parasites. As said above, a fecal sample aims at targetting the right treatment for the parasites which are actually present. None of the efficient drugs I know against any type of parasite is totally safe- cures for external parasites are more or less insecticides, and for internal parasites a certain toxicity is also present, depending on the active molecules.
Treating for parasites when you are not even sure there are any and which ones are there is like giving antibiotics to a human baby who has diarrhea :lol:
As said above, I would first focus on rehydrating the animal with an electrolyte solution like Gatorade and put him/her in a quarantine enclosure to check more easily the feces and food intake, and any signs of disease like quick weight loss, shedding troubles...
Then take a fresh fecal sample in a sterile jar to your local vet as quickly as possible (some protozoans don't survive more than an hour in the open air, so they may go unnoticed if you wait for too long), then once you have the tests results your vet should be able to give you a prescription targetting on the parasites which are actually present. You could do more harm than good by giving "blindly" f.e. Metronidazole or Fenbendazole if it is not necessary.
And last but not least, do whatever you can to minimize stress- enough hides for the newcomers, no unwanted noise, crude lights nor vibrations, no other pets lurking around the tank, and if your animals should be treated, you can in some cases inject the treatment with a syringe inside a prey then offer that prey to your leaftail with tweezers
