It sounds like you are really jumping to conclusions on the scratching.
Nope, not jumping to conclusions. Can't ever be sure how much background people have in common fish health issues, and ich is something I always am on the lookout for as it's so ridiculously prevalent due to the way fish are currently mass farmed/imported and stocked/kept in overcrowded conditions with mass filtration systems before sale to an unsuspecting hobbyist.
The fish were in qt for 3 weeks & showed no spots. They appear healthy, & breath normal & show normal activity.
Of course good signs, but does not eliminate the possibility that one or more may be carrying ich or many other parasites/diseases.
It is unwise to medicate & stress fish without proper cause.
I agree.
It is a myth that sudden stress causes an ick out brake.
That's not true (or just mis-stated). Any stressor can trigger a minor health issue into a major epidemic, especially in a closed system.
The parasites cannot live without a host after 3 or 4 weeks. I have had ick in the past & I know exactly what to look for. I qt all my fish. To heat a tank & add salt to a well established tank with happy healthy looking fish is unwise. Fish scratching can have many causes. We are also getting off topic. The original question was pertaining to problems with plants & fish with high kh !
More than likely it is possible they could have gill flukes but by now they would have had more problems than they do. After a while they will lay on the bottom & breath heavy & gasp at the surface. They do not show any of this, but I am keeping an eye out. My first choice would be prazi not any ick meds! But again until I see real problems I would refrain from meds. People tend to jump to conclusions & use meds for wrong diagnoses
Good, and agreed, but I personally think that you need to be careful about dismissing the possibility of ich/other parasite or disease completely just because your fish passed quarantine. It's definitely possible that your fish are having issues just from the dramatic water parameter shifts they've been experiencing. But that may not be the case, and, as I stated earlier, stressors such as the water parameter issues here often can trigger the outbreak of other problems that were carried in by one or a few fish but under control while their own immune systems were healthy... but now probably are not.
Ick is very obvious & ick should go away forever if people would just qt their fish, but most do not.
I wish that were true. But unfortunately, it's not. Ich is a tiny parasite and cannot be seen without a microscope. It is the lesions left behind by infestations at the point that the parasite has become an epidemic that are visible to the naked eye. The parasite can easily pass through quarantine if just a few individuals are present. Further, ich has proven to have evolved significantly over the years as it has gained resistances to many of the treatments that used to be effective.
In addition to ich, there are scientific studies underway that are identifying various retroviruses that seem to have literally injected themselves into the DNA of some of the fish that we commonly keep in the hobby... dwarf gourami and iridiovirus being one example that we know of.
...ALL of that to say- IMO you still need to keep a close eye on your fish and be prepared to medicate if needed, even after you figure out and correct your water parameter issues.