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Re: Kh, Ph, Buffering, and Baking Soda.
First off...these numbers look odd...you made it sound like you aren't yet
adding CO2 to the tank, but at KH = 4, to get a pH of 6.5 you would have
to have CO2 levels of about 40 ppm. Either I misread your post and you've
already got the CO2 going or there's probably another buffer system
interfering with these measurements; are you adding peat or phosphates to
the water?
Second, you ask if you are in danger of a pH crash when you start adding
CO2...well, the only way to know this is to know the current CO2 level,
since the magnitude of the pH drop when you add CO2 depends only on the
initial CO2 concentration, not on the initial KH. If you go from 1 ppm
CO2 to 10 ppm CO2, the drop in pH will be 1.0; having a higher KH will
not make this drop any smaller, but will just make the starting point
higher. (I won't go into this fully, but I have a long earlier post at:
http://www.actwin.com/fish/aquatic-plants/month.9910/msg00024.html
Unfortunately, this gives no simple answer for your system. Step one is
to figure out whether you really have 40 ppm CO2 or whether some other
buffer system is confounding the KH reading. If it's the former, then
you're fine -- no need to inject CO2, just leave things alone.
I suspect it's the latter, which makes life tricky. Ideally, we start
with a reasonable KH and add CO2 until it lowers the pH to the range
we want, then check the tables to make sure this is an OK range for CO2.
But the tables don't apply when there's a second buffer system, so this
won't work. I think your best plan is to just get a good CO2 kit, make
changes slowly, and watch the tank carefully. (Always a good idea when
beginning CO2 injection anyway!) Gradually increase the CO2 levels to
the range you want (remember it varies during the day) and watch the
pH; if it starts to get too low, add some baking soda. Keep careful
notes, and eventually you'll figure out how much baking soda your tank
needs to keep pH and CO2 levels where you want them.
Eric Tulsky