There is a professional breeder of Discus here in san francisco.........He has breeding Discus successfully since the late 60's...........Here is how his breeding tanks looks:
20-40g tall tanks........
One Brick standing in the middle for the pair to lay eggs.......
Small HOB filter, accompanied with an airstone.............
Bare tank, means no substrate on the floor..........
Temperature ranges between 78-82 degrees...........
As you can read from these quick notes, a large tank is not needed...........Simpler the better...Anything larger than a 30g can, and should, be used as a growout tank for young fry when they grow to fish shape(usually within 8-10 weks.........)The fish eat mucus off the oparents body, but baby brine shrimp seems to be a favorite when raising fry to fish.............