That's a lot of what ifs, and I've never heard of fish having an issue adapting. You can always use fake plants and other boilable decor in your quarantine tank to make it more homey/less stressful, and I even put a really thin layer of sand (I have extra) to cut the glare on the bottom, and dump it when I'm done. I think the temporary stress each new fish goes through in quarantine is negligable compared to the amount of stress your whole tank full of fish will go through without quarantining the newcomers...i.e., new fish is in quarantine and has ich/columnaris/what have ya, is treated and released to the main tank vs. every time you get a new fish, your whole tank (every fish you have bought and kept healthy up to date) is exposed to said potential diseases and has to go through treatment every time you bring in a new, diseased fish. This can also wreak havoc on your beneficial bacteria...say that fish came in with columnaris and you're treating your whole tank (instead of just a quarantine and one fish) with antibacterials, your bacteria die off, and you're now stuck in a fishy cycle months after you thought you were done with ammonia/nitrites. That's a problem I'd much rather have confined to a smaller tank, and I always keep extra sponges for my quarantine's filter in my main tank's filter so I have a constant supply of cycled media and can sterilize the tank/start over at a moments notice. I'd rather risk losing the one new fish to my whole established tank full.
