Just FYI, RO/DI is usually needed in marine since you will be adding to the water to buffer.
if you want to buffer breeding water for Discus etc you should use RO and add the proper mineral(salts) back to the water just as in Marine tanks.
(it is the most reliable technique if you want to modify chemistry of the water) as in the case of some sensitive wild strains(Altum, heckels etc)
for domestically raised Discus, angels etc .. you should be fine with tap (unless the fish are having issues with hardness of the water - some species may suffer osmotic regulation if the water is too far out of whack.)