Water chemistry questions are like filtration questions, food questions, etc., there are different opinions and philosophies and you virtually always see the same split of opinions between
don't change your water and
it's fine to adjust your water.
No matter what anyone says, there's no absolute, cosmic truth here, it's a matter of opinion and philosophy. The long history of hobbyists, breeders, and commercial aquaculture has clearly demonstrated that most fish (there are some exceptions) are perfectly adaptable to a reasonable range of water conditions. If this were not so, a lot of Florida fish farms would have either gone out of business long ago or their fish would be a lot more expensive. There are exceptions, of course, like some (not all) blackwater species that may have certain health issues in harder water or may be fine (within reason) but need softer water to breed successfully.
In other words, you can usually do what works for you, as long as it's within reason and not too extreme.
In fact, if you do enough reading and research into the different water types in SA or the real world variability of the rivers, streams, or habitat of some species, including angelfish and discus (as opposed to traditional assumptions), you find their native water is not always as soft, warm, or low pH as people tend to think. In many such cases 'evolved in soft water' should be amended to 'evolved to adapt to changing conditions over time or geological distribution.' Discus are a great example of this-- regarding what some call S. haraldi and others call S.aequifasciatus:
Definitive guide to Discus: part one - Practical Fishkeeping
"It is only found in clear water habitats, sometimes mixed with white water of pH ranging from 6.0-7.8, these conditions also making it the most easy to breed."
6.0-7.8. That's a wide range, and you can read the
2nd article in the series to see some of the other fish, including angels, that are often found with them. So-- not all discus actually come from very soft, low pH water, only certain species. Not all angelfish need very soft, low pH water, only certain ones (primarily altums).