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Re: pH and CO2

> I have been puzzled by an apparent rise in pH at night (as indicated by a
> Dennerle Visual Indicator)..  I decided to check what was happening with my
> regular test kits.  Yesterday morning, I measured a pH of 7.2 and a KH of 6.
> From the graph, I derived a CO2 concentration of 12 ppm.  Deciding that was
> a bit low, I increased the CO2 bubble rate -- too much as it turned out --
> so that by the time the lights and CO2 were turned off last ngiht, the pH
> had dropped to 6.8, indicating a CO2 conc. of 28.  But this morning, the pH
> was back up to 7.0.  And, thus, my question: If the plants stop absorbing
> CO2 at night and respire it instead, why doesn't the pH go down, rather than
> up?

Do you turn your CO2 off at night? I'll assume you add CO2 without a pH
controller. Your KH/pH for that morning was fine, as long as the pH goes
down a little in an hour or two. A pH of 6.8 is not a bad level either. My
CO2 levels sit in that area of 20-35ppm. Some folk's tanks have less, a very
few a little more. Don't let it get above those levels (30-35ppm) at any
time if you can help it.
But in answer to the question, since you turned off your CO2 that night, the
gas was simply returning to equilibrium. Same thing happens when you forget
to put the cap back on a soda pop bottle. It goes flat(losing all that CO2)
and it's pH also goes up.
 It will not have a pH of 6.8 unless you DO something to it ( continue
adding CO2 for example) after sometime has passed. It will rise if you don't
continue to add it to your tank, having plants using it up during the day
will only speed this process up. This is not a bad thing as long as when the
plants have light- they also get CO2 in a good range. When a plant first
"wakes" up in the morning, they don't use as much CO2 as the midday (so it's
okay to have lower CO2 at the first part of the morning). Kind of like us:)
Some folks need some coffee in the morning before they start going.

Some folks have even added coffee beans to their tanks if you would believe
that. Namely for the breeding of SA fishes, not for groggy plants:) Don't do
this for the plants please!

 Some will argue that it's best to leave your CO2 on all night some will
argue to turn it off at night. Both methods work. I turn mine off at night
personally though. My pH 's will rise at night and go down to the lowest
level right before the lights go off. The reverse set up were the CO2 is
left on 24/7 the opposite will happen where the pH will be at it's lowest in
the early morning when the lights come on and the highest right before the
lights go out. Try and shoot for a range of pH's of about 6.8 to about 7.2
throughout your day. That would give you a CO2 of 12-28ppm. Nothing wrong
with those levels.  Don't worry about a single reading of your pH. It's
impossible darn near to hit a perfect pH all the time etc and it's plain
unnecessary. Same goes for Fe, NO3, P, K etc.
Just try to keep it in a "balanced range". This is the "ultimate truth" in
planted tanks. Relax and enjoy:)
Tom Barr        -Still (some say permanently) puzzled:)

> A puzzled beginner,
> John T. Fitch