you can't keep discus in a 12"x12" tank no matter how tall it is. with such odd dimensions, I think you'd be best off with a bunch of tetras. you'd have to look around to see what kind of small fish would be good with 72" of water overhead. I know most cories don't go deeper than 24" down and so would be pretty stressed at having such a tall water column.
glad I'm not the one that has to vacuum the gravel.