I'm not sold on QT and treating all current fish if truly asymptomatic, but I would def consider doing one of these https://aquabiomics.com/
Follow along with the video below to see how to install our site as a web app on your home screen.
Note: This feature may not be available in some browsers.
I've had a similar experience. Almost all of my fish have been in my tank for over 5 years some more than 8 and are all strong and healthy. I have tried adding a few over the last year or so and they last for a few weeks or so looking strong and then just are gone. I stopped trying to add fish.Dude, I'm asking for solutions. I understand your point which is the reason I'm here.
I looked up Jay's posts and I didn't see his step-by-step. Do you have a link?I would put your current fish in QT and treat per Jay Hemdal's outline. Let your DT go fallow. Then, I would try a different fish store and QT them also.