I doubt you have parasites or at least I doubt that that's your original problem. They sold you something that you probably don't need but can't hurt at this point. If the water sample was from a tank that is not cycled and wasn't taken right after a large water change it would have shown ammonia. If it was taken right after a water change it is a useless sample. You really should get that tank cycled before you think about doing anything else. Large daily water changes until the ammonia level starts going down.