I believe from the many post's on many forum's,,that enough people struggle with keeping soft water fishes in unsuitable ,hard water,and vice versa that some weight should be given to the wise suggestion's of keeping softwater species in soft water,and fishes who enjoy the mineral content in hard water in more alkaline condition's.
Life for the fishes, and hobbyist, is much easier this way.
Those who do not heed the advice offered,, often spend considerable $$ replacing/medicating sick ,dying/dead fishes.
Been keeping guppies since I was in high school,(40 year's ago) and the large majority of them will nearly alway's fair better in hard ,alkaline water of 10 + degrees hardness with one male to three or four females.
Tetra's are just as likely to do poorly in hard water, and appreciate soft acidic condition's not withstanding those of this species that do well in broader range of water's, or those few who claim to keep them successfully in hard water (How hard,how long?).
Will alway's be those who keep fishes outside their comfort level's but they are in my view becdoming fewer and hopefully this is a result of seeking knowledge BEFORE purchasing the fish as opposed to plopping them in the tank and," wait and see".
Can keep and Eagle in a bird cage,,Raise a child in a closet,, but they will not thrive.