Fish kept in environmental conditions for which they are not naturally adapted are under constant stress. This leads to a breakdown of their immune systems, and they are then highly susceptible to diseases that otherwise would not have bothered them.
The idea that a fish can evolve for millions of years for particular water conditions (pH, hardness, specific trace mineral content, etc.) and then "acclimate" to completely different conditions over the space of a few months (or even over a few generations of breeding) is false. The fish in question may survive, depending on the durability of the species, for quite a while, but it will always be fighting osmotic pressures and other environmental factors for which its systems are not adapted, and this will lead to a shorter lifespan. Sometimes much shorter.
Chemical treatments which lower the pH will not really address the mineral content issues, especially if you keep up a healthy maintenance routine and do regular water changes. So, if you really want to keep softwater species and keep them healthy I would recommend getting an R/O unit (a low capacity unit is not that expensive on-line), and then using that water 50/50 with your tapwater for changes. This softer water will be more to your angels' liking. When I lived in LA and had discus, angels, and apistos, this is how I kept them happy.