Whenever discus & angels are mixed the discus suffer, simply because they are less aggressive feeders, so they either slowly starve or suffer from poluted water from "overfeeding"
Is an absolutely rubbish statement, whether you breed or just keep angels or discus, its always worth having an open mind.
Discus arent always the less aggressive feeders, it just depends on how settled into the tank they are, mine are by far the more aggressive feeders and will boot the angels right out the way to get the food first.
If the discus are suffering from starvation then it is because the person feeding them isnt giving them enough or the right food.
If the discus are suffering because of poor water quality then it is because the person in charge of the tank isnt doing enough or big enough water changes, its an immature tank, they are over stocked and so on, not because they are keeping discus and angels in together...
That said, I keep both angels and discus together in my 340L tank, the discus are both larger and smaller than the angels, i have mixed sizes of both species, both discus and angels are starting to spawn (not with each other!!) and still there are no problems. They get one or two good sized water changes a week (have two large tetratec filters on the tank, one does 500L the other 250L or more i think) and they get fed Novobits when i get in from work then when they have all finnish a big pinch of these then they get a couple of cubes of either beefheart, bloodworm, mysis, krill or greenfood and then they pick at the novotabs dropped in for the clownloach.
The water quality is always spot on, the fish dont fight (one exception of a new discus which i am going to remove as it is actually female not male LOL and my small female thats ready to spawn doesnt like her).
So long as the water parameters are right for discus or as close as possible then discus would be fine in the tank. I wouldnt advise using tap water unless you are lucky enough to have ridiculously soft water like on the south coast or in some parts of wales... I personally use RO water with the Tropic Marin Pro Discus Remineraliser.