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Re: Aquatic Plants Digest V2 #771

> Date: Sat, 14 Jun 1997 14:24:43 -0500
> From: Daniel Hawley <dhawley at iamerica_net>
> Subject: co2 and aeration
> I have a 100 gallon gal plant tank that I have been running for
> several weeks. I use most of the Dupla method and their equipment and
> supplements. I presently use a Neptune controller for the co2. I have
> one problem my ph has been dropping to 6.50 during the night and
> slowly rises to 6.80 after the metal halides come on. The set point
> for the co2 is on at 6.80 and off at 6.75 1 have been considering
> adding another timer to turn a air pump on when the ph drops to 6.60
> and to turn off when the ph rises to 6.70. I believe that the aeration
> will only be on when the lights are off during the period that the
> plants also use oxygen. please let me know if this is a good idea.

No, not a good idea.  Removing CO2 at night only to have to add it
back in in the morning is wasteful and counterproductive. 

When you use an electronic CO2 controller, you need to have enough
carbonate hardness (AKA "KH", alkalinity, acid buffering) so that your
water without extra CO2 will have a pH higher than your set point. It
sounds like you have a relatively low KH and, in turn, not much CO2 is
being injected during the day.  You have an expensive system that is
not doing you much good.

For example, we have a carbonate hardness (measured by the Tetra KH
test kit) of 5 dKH (degrees KH). This gives the water an "equilibrium"
pH of around 7.8 with the typical 3-4 mg/l of dissolved CO2 that
occurs naturally in an aquarium. The CO2 controller then injects
enough CO2 to bring the pH down to 7.0, giving us a decent amount of
dissolved CO2 (15 mg/l). 

This water chemistry is a kind of a "tug of war" with KH trying to
raise pH and CO2 trying to lower pH.  The controller acts as a
mediator, making sure that the CO2 wins just enough to produce a
specific pH.  Of course, CO2 (a natural born loser) is always trying
to diffuse into the air since there is "too much" in the water, so the
controller has to keep adding more back in to maintian the pH.  If
you don't have enough KH, other acid sources in the tank (natural CO2,
H+ from nitrification, etc) overpowers the the acid buffering, taking
your pH below the controller set point, rendering it ineffective.

Use one of the CO2 charts available in the Krib or on the web. Decide
what pH you would like your water to be.  Then decide how much CO2 you
would like to inject (15 mg/l is a good target).  Find out what value
of KH coincides with that pH and CO2 level. 

Now use sodium bicarbonate (NaHCO3, baking soda) to increase your KH
to the indicated level.  One teaspoon (about 6 grams) of sodium
bicarbonate per 50 liters of water will increase KH by 4 degrees and
will not increase general hardness.  If you would like more general
hardness, two teaspoons (about 4 grams) of calcium carbonate (CaCO3)
per 50 liters of water will increase both KH and GH by 4 degrees.
Different proportions of each can be used to get the correct KH/GH
balance dictated by the fish and plants in the tank.  Since it is
difficult to accurately measure small quantities of dry chemicals at
home, a test kit should be used to verify the actual KH and GH that is

With increased KH, you will find that the controller will now maintain
the pH at the set point, +/- the built-in hysterisis (0.05 ?). And,
hey, your plants might grow better, also!

George Booth
Need info?  http://www.frii.com/~booth/AquaticConcepts.htm