CO2 Swings

>Also, recently I put together a C02 injection system-luckily found a near
>new used regulator and bottle for a very reasonable price-but now I'm
>faced with a bit of a dilemma when doing water changes.

>The PH of my tank has settled nicely down to 6.8-7.0 but the PH of the
>water used in my weekly 25% water changes is up around 7.4.  I'm worried
>that this might be too much of a shock on the system especially with some
>of the more sensitive plants prone to metldowns such as crytocorynes.

I have a similar situation: I start with dKH 3-4 water with a 7.5 pH and a
low CO2 level.  My CO2 reactor will crank up the CO2 level to 15 mg./l and
higher, so if I lower the pH of the change water to 7.0 (with acid) and
then add the water, the pH will drop even further when the CO2 level goes
up (forcing one to add bicarbonate to raise the pH back).  Before I figured
out the KH/pH/CO2 relationship, I used to do this for a couple hours on
Saturday mornings--trying to hold the pH around 6.8-7.0 as I changed the
water.  I don't bother with this anymore.

Neither my fish nor my plants (including crypts) seem to mind pH swings in
the 6.8-7.2 or even 7.4 range.  Amano (or his people) have made the same
comment somewhere (in answer to an APDer's question, I think).  I've raised
the KH a little in my tank and believe this has reduced daily pH swings
based on plant usage of CO2.  (It is at least clear from the KH/pH/CO2
chart that a 6.8-7.0 pH swing requires a much greater change in CO2 levels
as the KH increases. In other words, if the pH swings haven't reduced, the
CO2 plant consumption (and/or out-gassing) has increased dramatically.) 
And I haven't had a crypt meltdown since I started using decent amounts of
fertilizer in my tank several months ago.  

If you wanted to minimize CO2 swings, you could add the change water slowly
(perhaps in two steps) which would allow the CO2 reactor to bring the pH
down as you add the water, but I don't think a 25% water change with a pH
of 7.4 will adversely affect your plants.  If you had high KH levels in the
change water, it would take more CO2 to bring the pH down to where you
want.  It doesn't sound like this is your situation, since you start with
7.4 water and your CO2 reactor is settling in around 6.8-7.0.  My guess is
your KH is fairly low.

Tetra's KH test kit has been a good one for me.  (It's the only Tetra kit I
continue to use.)