Hmm...it doesn't make sense. I've kept many angelfish, which are typically soft water species, in my tank and they have never had problems due to sickness. I don't understand PH.
Many angelfish have been aquarium bred for over 100 years, so may be more adaptable.
I first started keeping angels around 1960
This is "not" the case with bush fish, they are still barely one step from their wild conditions.
But
As an example
one of my favorite species, is Uaru fernandesyepezi.
This fish comes from the river Atabapo in Venezuela that has an average pH of 5, soft, mineral free.
It has only been in the aquarium hobby about a decade.
Expecting this species to adapt to my hard high 8.2 pH water would be a bit delusional.
And most reports of those who tried to keep them in adverse conditions, have been quite disappointing.
For me, because I always try to acquire at least half dozen of any species, at minimum $60 each, ($360 for the group, add shipping of at least $50 too $100) and having them flounder and die would be a poor investment.
So even though I have wanted them a decade, unless I had perfect conditions for them, I won't get them.
Trying to chase pH, unless you are very adept at chemistry, is not usually recommended. (and it may also have to do with your waters hardness, conductivity and TDS)
The better action, is to try and only keep species that live in your water parameters and conditions.