I think the biggest advantage of a triangular aquarium,over the aquariums of other shapes,is that it is deep enough to allow beautiful decorations using live rock and live corals.Another advantage(especially for a 90*,45*,45* aquarium) is that fish should equally move in all directions while the viewing pane is bigger than the other two panes(something that is not the same for a cube aquarium).As for cleaning the corners,it may be difficult when approaching them,but you could clean them using your hands and a 100%cotton towel(so as to be soft).