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"Are my Re: CO2 levels dangerous yet?" and other questions

I accidentally gassed my fish when I first put the diffuser stone in the
outlet of the powerhead - the absence of bubbles slipped me up.  The Ph
went from 7.6 to about 6.2 in two hours.  According to the table in
http://www.frii.com/~booth/AquaticConcepts/, my predicted CO2 level was
240ppm.  The fish were gasping at the top of the water.  I immediately
stopped the CO2 and put a lot of bubbled air into the powerhead
venturi.  The fish returned to normal about an hour later. Needless to
say, I have been more careful since.

I add CO2 using a single stage regulator and a Nupro 4MG2 valve which is
way oversized for my 135 gallon tank. According to the table, my daily
CO2 level is between 24 to 47 ppm.  The fish don't show any distress and
the plants bubble oxygen very happily.  I start with a 7.6+ Ph and 8 to
9 Kh water and adjust the additions to hit a 6.9 Ph. It takes about 6
hours to go from 7.6 to 6.9. The only fish that seem to mind the Ph
swings  are some rift lake cichlids that have eluded my nets and traps
over the past months.

For regulation. the Nupro valve is just barely cracked - I regulate
additions off the downstream gauge setting. As long as the needle valve
is not touched or jarred, you can hit the desired Ph level fairly
reliably by repeating the pressure gauge setting.

The night before the weekly 50% water change, I cut the indicated CO2
flow setting in half.  This gives me a 7.4 Ph.  I feel this Ph is close
enough to make water changes into directly.