The PH level in my tank is always 6.4, my tap water is more like 7.4. I have few large pieces of driftwood in the tank which would be responsibal for the drop in PH but is the difference in PH ok for my fish when im doing a water change???? Should i think about adding something to my tank like a piece of coral so that my tanks PH is closer to the PH in my tap water? Also does adding salt raise PH levels?

what i do , and as you may know i got a lot of driftwood, is use kH+ and keep my kH at 7 at all times.
our tapwater is very soft.
By keeping the kH steady at 7 my pH will do the same, and stay steady at 7.
I learned this trick from my pleco dealer, for plecos are sensitive to changes in pH.
This works for me...

have you left the tap water to stand for 24-48 hours before the test? it would also depend on how much water you change on each water change. i certainly wouldn't like to do 50%+ water changes. Noddy seams to have a good solution to the same problem, and as you say crushed coral may help raise the tank PH a little. are the fish affected when you do water changes?

If you have got quite a bit of wood in your tank then this drop in pH is quite normal, I have exactly the same thing going on in my tank. Large changes in pH is obviously stressful to fish, however if you carry out regular water changes then you shouldn't need to carry out huge water changes anyway, so you're pH change dramatically.

i normaly do about 30% each wk, maybe a little more. They dont really seem to affected by the whole process, maybe the more timid are a little stressed but mind you i do shove a big plastic pipe in to the home, suck water out and tidy up any thing that needs doing so i guess a little stress is ok. Seeing though its no biggie i might just leave it for now but when i get round to it might put some crushed coral in my filter just to raise it a little so its closer to my tap water just incase i need to ever do a huge water change for what ever reason. Thnaks for the info guys.