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Re: Airstone anathema

>From: "A. Inniss" <andrewi at u_washington.edu>
>
>> >From: Elizabeth Worobel <eworobe at cc_UManitoba.CA>
>> >
>> >There are only two instances where it is 
>> >beneficial ... the first is when there is a daily fluctuation in pH
which 
>> >indicates that the demand for CO2 by the plants exceeds the abil
>> >ity of the water to absorb it (of course in this case aeration will
also 
>> >work) ... 
>> 
>> In your first case, aeration will help.  CO2 injection will help a lot
>> more.  CO2 injection will also prevent biogenic decalcification in tanks
>> like mine that don't get frequent water changes.
>
>Why would aeration help?  From what I've read, aeration tends to
>raise pH (though I haven't yet read an explanation as to why)  and
>pH fluctuation in a plant tank already means a rising ph in the
>day.  Could you explain this one a little more fully? Thanks!

CO2 dissolved in water is at least partially present in the form of
carbonic acid (CO2 + H2O -> HCO3 + H+).  Carbonic acid lowers the pH, and
is utilized directly by the plants, raising the pH as they remove the acid
from the water.
Aeration will help control the pH in this case by driving off CO2
concentrations greater than what you'd find at atmospheric equilibrium.
Since often a tank without aeration and with little surface-air interaction
will increase its CO2 levels at night when the plants aren't consuming CO2
(they actually produce a little CO2 at night) and the same tank will drop
in CO2 levels during the day because the plants will consume the CO2 in the
water, the pH tends to fluctuate.  
Automated CO2 injection systems turn on and off CO2 delivery based on the
pH of the water to control the pH.  Other non-automated CO2 injection
systems rely on alkaline buffering of the water to keep the pH relatively
stable as CO2 is added and consumed at unequal rates.  There is still a pH
swing, but not as much as would be there if the water had no alkalinity.

If you have only plants that can take their CO2 needs from bicarbonates,
(Ca(CO3)2 + H20 -> CaCO3 + HCO3 + OH-) they can grow quite happliy by
consuming carbonate alkalinity for their CO2 needs (much more slowly
though, because the reaction above requires energy).  The extra OH- ion
left over will cause the water to rise in pH, though.  If you change your
water regularly, this shouldn't be a problem.  In this type of tank, the
CO2 levels maintained in the water by aeration are quite sufficient.  The
required alkalinity for this type of tank will probably keep your pH at
around 8.0-8.5, however.

If you have plants that cannot break down bicarbonates to gather their CO2,
they will have a very difficult time competing in this type of tank, and in
my experience, you set yourself up for an algae problem.  This is where CO2
conservation/injection techniques can come in particularly handy.  They
also work quite well on tanks that have carbonate-consuming plants.

Dupla's materials on aquatic plants suggest that many aquatic plants come
from streams with carbonic acid-rich springs exiting into the streams.
They also suggest that stream areas directly below rapids and other highly
aerating natural occurances generally don't grow plants quite as rapidly as
areas with less surface-air interaction.  My own observations concur with
this, both in the wild and in the aquarium.  I've found that CO2 injection
in my tanks can be quite useful in controlling algae outbreaks if I'm
keeping the rest of the nutrients at correct levels.

David W. Webb           Enterprise Computing
Texas Instruments Inc.  Dallas, TX USA
(972) 575-3443 (voice)  MSGID:       DAWB
(972) 575-4853 (fax)    Internet:    dwebb at ti_com
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