I guess my confusion is if there are fish diseases, how would you know from QT'ing the plants? Would there be some kind of fishless symptoms to look for? Would having the plants in a separate tank for 2-3 weeks eradicate the potential disease without any kind of treatment? I don't mind leaving them in the jug for a while but would I need to change the water during that time? I have potassium permanganate (in the form of Jungle clear) that I could soak them in for a while.
No, you would never see the diseases without fish present. Maybe an example will illustrate. Ich (white spot) can be attached to plants, wood, etc. that comes from a tank with fish. Putting the plants in a tank with fish could result in the ich hatching out and infecting the fish. I say could; fish can fight this off if healthy. Keeping the plants in a tank without fish for a week would mean the ich, if present, would hatch out, and not finding any host fish, die within 24+ hours. End of ich. Similarly, plants in transit for 3+ days would likely have the same result. Ich needs to find a host fish within roughly 24 hours.
I am not expert on disease to the extent that I could suggest which diseases might be comparable. Ich and Velvet can transfer like this. Others may too.
The other thing about quarantining plants or fish is that one can never be certain of completely avoiding disease. I have had disease appear in new fish that I had for several weeks, once in the QT. We do what we can as a precaution, but preventing stress is still the best medicine. Stress causes 95% of all fish disease, if a pathogen is present.
QT plants is something I rarely do. Sometimes I may drop them in my permanent QT tank, sometimes they go into the intended aquarium. You want them in a proper environment as soon as possible, meaning sufficient light, proper water, and nutrients. Plants can sometimes withstand rough treatment, but this shouldnot be prolonged any more than necessary.
Potassium permangate is not going to benefit anything. This wouldn't for example kill ich or parasites, or much else. Anything strong enough to really kill algae or snails for example will inevitably harm the plants, most plants anyway. Not worth it.
Byron.