Aww fish don' t really care about pH... huge swings are very natural, especially for smaller bodies of water (not so much for rivers...)... they don't care about hardness either (altho it may effect development of some eggs)
After finding a few more hobbyists that I respect using this technique, I'm a "squirt-and-dump " guy now, infact one (very very experienced) breeder I spoke to at the AGA had some hard to spawn rasboras go at it after getting fish from a fellow hobbyist and a technique much like this. Absolutely no acclimation, right into the tank:
Take home message is that fish don't care about pH or TDS differences, its written in every book becausae it was in the books before them. The only thing that'll give 'em a good lick is severe temperature shock which is really hard to do as water has a huge thermal interia.
Since I'm soapboxing a bit I'll continue if you'll let me, loads of what we do, and the reasons we do it are based on bad science and poorly thought out protocols and "conventional wisdom". So, if something is working for someone, that is a fact, if someone tells you what you're doing is wrong, that is theory, in the game of science facts drive theory, not the other way around
So all this hypertechnical bickering you see on the mailing lists and books, ignore it, folks are just blowing smoke
Experiment! Find a system that works for you..