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Daily pH fluctuations

Oh boy, I'm getting around to seriously considering CO2 addition to my tank,
but before I decide (I know, read the archives) exactly what to do, I would
like to get some input.

The pH is 6.7 in the morning before the lights go on.  (3 watts/gal)
It is up to 7.5 by the evening when the lights go out.
My plants are growing like crazy, despite the lack of supplemental CO2.
I give the tank trace elements and potassium and magnesium with every 20%
twice weekly water change.  The iron level is not above 0.1mg/l.  Nitrates
are just barely measureable.  Phosphates inconsequential.  No significant
algae growth.  (a little on the older leaves).  SAEs busy doing their thing
all day.
General hardness is now 4 (Increased it a bit).  Carbonate hardness of the
water I put in is 3, but when I measure it in the tank at the end of the day
it is maybe 1.5.  This concerns me. I assume it is because the nitrate is
not zero.

I've been considering all the DIY recipes, and now I have a Dupla catalogue
to intimidate myself with.
My understanding, re:DIY is that CO2 is bubbling into the intake of the
canister filter 24 hours per day.  The Super dooper (expensive) Dupla system
has a computerized feedback which controls CO2 flow, and provides it as

I'm willing to do some more sophisticated setup than the soda
bottle/champagne yeast system (hence, will go back to the archives),  but my
concern regards to the pH at night when the DIY keeps bubbling CO2?  Or how
can I fine tune a system if I get something that is more sophisticated than
the bottle, but not as laboratory-like as the Dupla (which I can't afford
anyway)?  I'm afraid that my pH will go too low at night.  (Also, my husband
has a real problem with having a gas canister in the bedroom.  I will have
to disguise it, so I need to make a very discrete set-up.)

At this time, my fish are doing fine.  I only became alarmingly aware of the
pH shifts once I installed a pinpoint pH meter.  I understand that some pH
change is not a bad thing.  But, is almost one whole degree safe in the
longrun?  None of the other tanks does this, but they are not as heavily
planted either.

G. Kadar
p.s. I've sent off the paperwork for the Aquatic Gardener's magazine, and
the back issues too, because I can read those in bed.  I cannot sit in front
of a computer for hours on end reading through all the archival material.
In too much pain.  Sorry if this post is very repetitive.  If I can get a
couple of off-list responses that would suffice.