A display tank is larger, has rock and other fish. My fish don't chase each other around because I've provided enough space so they can all establish their own territories. Putting a fish alone in a relatively small tank is more stressful. I've observed in my tank that fish feel more comfortable in numbers.
And since this thread is about ich. Why would adding an infected fish affect all the other fish in the tank? If they're happy and healthy it won't matter. And once you put the new fish in, once it's happy it'll fight the parisite and that's it. (doesn't take 6-8 weeks in a small 20 gallon tank)
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