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Intro and CO2/KH Mystery

I started my first planted tank about 2 years ago in Northern California.
It was a shining success thanks to the APD archives and The Krib.  It kept
me from making (some) typical beginner errors.  And some things you just
have to learn the hard way- you can only read so much and then you have to
put it all together.

The single biggest mistake/regret I made with my first tank was I bought a
50 pound bag of Lapis Lustre for my substrate, which was loaded with calcium
carbonate.  Dealt with the constant water changes to try to keep the GH and
KH under control- pain in the butt.

I have now moved to Dallas, TX and I just started up my second planted tank
adventure (45 gallon).  The substrate is Fluorite this time around, but I
added in some Dallas mined quarry pea gravel, maybe 10%, for some slightly
larger pieces.  I  figured since I am starting the plants first without
fish, I'd really get the CO2 up there- I put 2 DIY yeast bottles both
connected to venturi inlet of my powerhead.

Out of the tap, the KH is 3.5, pH is 7.5-8 range (Tetra).  After running the
CO2 for a few hours, the pH had dropped to 6.2 or 6.4 (Red Sea)- that was
fine and dandy.  Then a troubling thing happened.  After about 36 hours, I
checked the pH and KH- the pH was still low 6 but the KH was now 6!!!
Almost doubled in less than 2 days.  Holy Cow!!!  I rechecked these numbers,
and used 2 different tests for the pH.  How could that be?  And that also
means the CO2 is waayyy over 100mg/L, getting close to 200.  Is that
possible?  I didn't realize 2 bottles for a 45g would kick it up THAT high.
I totally disconnected the CO2 and checked all 14 hours later in the morning
after lights had been on for almost 3 hours: pH was STILL low 6 and KH 6 and
GH 10 (tap is 6).  Just what is happening- How could the pH still be so low
with no CO2 injection for 14 hours?  Is there a rock I might have added that
makes the water acidic?  The plants are still covered with bubbles this
morning- they appear to be doing fine at the moment.

I just want to kick myself.  I never considered the gravel from North Texas
would have high CaCO2- there is no beach here.  Maybe I was running too much
CO2 in there and creating too much acid, reacting heavily with the CaCO2 in
the gravel.  I'm just totally flabbergasted.  The Lapis Lutre didn't even
shoot up this fast and that stuff is full of shell remnants.  I don't THINK
there would be shell remnants in Dallas quarry, so what type of rock might
it be that is highly reactive?

I have to pick all that damn quarry stone out of my Fluorite and start over.
I'm so depressed.

Andrea Jung