you dont want to change the PH instantly so a water change over a period of 30 mins will be fine, regardless of percentage replaced.
ok will do 50% water change tues morn then add new water conditioned and treated. then white spot . why the need for 30 mins
just so the PH difference doesnt happen very quickly, it will give the fish time to adjust...it might be worth taking an hour over it just to be safe.
maybe i have been doing a water change wrong then too quickly. ive been sucking out water using a syphon claening the gravel with everything off in the tank light and all. then filling a bucket of water to the correct temp using a kettle then conditioning that bucket and pouring it in the whole process has took me about ten minutes how could i make it last an hour. im sorry you must be fed up of me by now. im a big newbie though.