Many years ago I tried that. Just passive observation in QT, good nutrition, and I never treated with any chemicals unless I saw obvious symptoms of disease. My main DT was a 400 gallon with batfish, various angels & butterflies, and a show-sized Moorish Idol. All the MI really ate in the 5 years I had him was live blackworms, so that was a staple in both QT & DT. Everything was going great, until about 10 years in a Hurricane came which caused an extended power loss. (I didn't have a generator back then.) I was able to keep temp under control and the tank oxygenated using several battery powered air pumps, but it was at this time that I discovered my DT had latent ich. :( Secondary infections then started to break out, and I lost almost all of my fish. It was like this chain of events rapidly unfolding in front of my very eyes, but I was powerless to stop it. It was an emotionally devastating experience for me, as some of these fish were 10+ years old. I KNOW my fish would have probably lived if the tank didn't have ich, because other local hobbyists (who never saw ich during the entire ordeal) managed to save most of their fish despite operating under the same conditions. It was this experience, more than any other, which finally changed my mind about "ich management", and I decided to prophylactically treat in QT moving forward.