Scheel had what might be considered today some unorthodox opinions regarding water chemistry. He felt differences in salinity, or total dissolves solids matters very much, but that pH changes could be safely ignored. Given this observation was made over many decades of working with killifish it's a pervasive argument. Here are the relevent passages from Rivulins of The Old World:

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"If you move rivuline individuals from a water containing much dissolved mineral matter to water containing little such matter, these individuals probably soon will be swimming at he surface and they soon die. If you inspect the gills under a microscope you will realize the reason for this. You will find the gills completely ruiined. The many brittle cells of the gills burst shortly after the individuals bhad been placed in water of low salinity, because of the large difference in osmostic pressures. If the fishes are moved from water containing very little dissolved minerals to water containing much matter nothing will usually happen.

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I spawn my non-annual species in rather small tanks (about one gallon of water) and the tanks contain nothing but a mop and the water. Females are separated from males for at least one week, while both sexes are heavily fed. Only one female is placed with the male in the tank, because one female may eat eggs when the other is spawning. As my two types of water have similar salinities I can move the individuals right in and right out of one type of water without any harm. Differences in pH values, even from 5.0 to 8.0 have not produced any harm when individuals suddenly are moved from one type of water to the other

For what it's worth I havn't measured pH in 25 years - I ignore it and only pay attention to total dissolved solids and temperature.

If you want to read the "Scheel letters" - correspondance between Scheel and various aquarists, have a look at at http://new.killi.net/articles/by_Author/Scheel/