Hi neupane00....IMO , if a fish doesnt show signs of sickness or disease, to medicate them is totally unnecessary ....if the qt tank is fully cycled, warm, and filtered, 2-3 weeks is a great time to know if your fish are sick or carrying anything with them.....the the acclimation to the new tank is just as important...you dont want a
pH/temp shock to stress them that can cause the illness's you have experienced....
ive read a bunch of your threads prior to this one, and stress from adding the cardinals could have started this all..then with the addition of a bunch of different meds, could have
LED up to the rest of their issues...
do you have a reliable
test kit that will test all the parimeters yet? of course medicating your fish is your choice, but IMO you will have many more issues by medicating a healthy fish for "just in case" purposes than having a nice healthy qt tank