It depends. If you get wild caught or high end discus they are more sensitive than locally bred discus of assorted varieties. I got most of mine at the LFS I work at. I let them settle in before I got them. They came in as 'assorted discus', these are what I have found to be the hardiest. Angels are now pretty hardy. I currently have discus but no angels. I had angels for a little while a little while ago, before I decided to do something different with that tank. They settled in quickly, learned real quick what the tube coming into the tank meant (I used an UGF tube to put the sinking NLS pellets on the bottom by the corner they all hung out in). They started growing very quickly. Angels are hardier. Both need high water quality and a stable pH, as with any fish. I do not treat my tap for my discus, which is well water and sits at about 7.8.