While I do completely agree with everyone saying the 20 is to small for the aro, he did say that his larger tank broke. So the 20 is more of a holding tank, not a living tank. Since hes selling it on craigslist, he can clearly no longer properly take care of the aro so hes trying to get rid of it. So think of all the possibilities out there.
As far as the one dude goes saying theres nothing wrong with this, please sell your tanks and never buy fish again.
I think he was saying that there is nothing wrong with the aro being temporally housed in that tank. (which I agree with, though it's not ideal)