I don't really know what fish can take as far as fluctuation in PH but I know it isn't good for them. My local tap is 6.2'ish although my 55 Gal has a PH of 6.8 due to the substrate I use. Also due to the fragility of my fish I choose to do small (5 Gal) water changes every 1-2 days. If (like today) I do feel a need to do larger water changes I typically add a bucket of water each time I remove one, and do this multiple times. This will allow the PH alteration to develop slowly allowing the fish to acclimate.