Water Quality

Thanks to everyone who replied about my sand question.

Now for another.  The water in my area is hard (250 ppm) and has
a high pH (8.2).  I have had good plant growth, but I am adding 
a DIY CO2 system later next week.  My problem is is that I make large 
(50%) water changes weekly and want to know how this is going to affect
my fish after I get the CO2 system running and the pH is accordingly much
lower in the tank then in the new water.  
Should I switch to RO or Deionized water?  What are the advantages of 
each?  I have seen the Aquarium Pharm. Tap Water Purifier, but am leary
of using it because from the posts on the net I would only get ~100 gall.
before the cartridge needs to be changed.  That is only about two water
changes for me.  The RO unit sounds nice, but it's expense and the waste
water it produces bother me.  Also I have heard that when used in conjunction
with hard tap water, the membrane will need to be replaced fairly often.

Any help will be appreciated,

Eric Schoville
schovill at expert_cc.purdue.edu