In the Maroni river in Guiana where keyholes are naturally found the pH average is around 5.5.
I'm sure, since keyholes have been in the aquarium trade forever, this low pH has become unimportant from generations bred in more mineral waters.. But I would guess, it wouldn't be harmful.
I first had them in the late 1950s, kept in hard Milwaukee water, and they did fine, but I wouldn't be surprised if I had not been ignorant of health benefits of tannins or their soft water preference (and could have done something like added rain water, they would have done better).