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RE CO2 causing a rise in KH

Tomoko Schum wrote, in response to my comment that . . .CO2 won't lower
or raise the KH.  

"Scott, This is not what I was suggesting.  What I said was that the
water and wastewater board in my town rerouted my main water source to
the well with a high KH value and caused the KH of my tap water to
rise.    As a result my dissolved CO2 level has also gone up and caused
my fish to go belly up.  The pH value of my tap water must have gone
up, too, (sorry I did not check that one) since I did not see the pH
value of my tank water shift down like it should have with more
carbonic acid in the water.

After slowing down my bubble rate yesterday morning, the pH value in
the evening (one hour before the tank light turned off) came out to be
7.0.  This morning some of my fish were again gasping for air but not
as bad as yesterday morning. The pH value of my tank water this morning
was between 6.6 and 6.8 (hard to tell with AP's color chart.)  I
suppose I need to start aerating at night."

Sorry for the confusion, I mistook your mentions of KH as suggesting
that the KH had anything generally to do with hgow much CO2 would or
could be absorbed in the water.

If you run the CO2 day and night, you can probably expect the pH to be
higher in morning than later in the day.  In the early morning, the
plants haven't yet started consuming a lot of CO2; by late in the
afternoon or evening, they might have depleted the available CO2.  All
this, of course, depends on the CO2 supply rate, amount of plants,
ability of plants to use the CO2, lights, plant health, etc.  But a
variance of 6.8 - 7.0 between morning and night could be nothing more
than the difference between plants depleting the CO2 by end of day.

Based on your previous comments that your KH is 6, then to lower pH to
6.8-7.0 (the levels you cited) requires a CO2 level of 18 - 28.5 ppm. 
Even at 29 ppm, that should not be a fish-poisoning level--some fish
keepers maintain higher levels than that.  Of course, allowing for
margin of error in the KH and pH tests, given typical home test kits,
the CO2 level could be higher (or lower); it varies greatly with small
changes in the pH.

When I was uncertain of my measurements of pH, I bought a Hach brand
CO2 test to verify the CO2 levels that I imputed using KH and pH
measurements.  The Hach test is pretty accurate when used carefully.

Good luck,
Scott H.

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