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Re: Airstone anathema

>From: Elizabeth Worobel <eworobe at cc_UManitoba.CA>
>
>To suggest that airstones are somehow 'bad' for aquatic plants is wrong, 
>wrong, wrong (to repeat a recent post)! There is clear research which 
>indicates that photosynthetic rates in aquatic plants increase with 
>increased rates of aeration ... the reason is that increased aeration 
>increases water movement and therefore decreases the thickness of the 
>boundary layer. It is this boundary layer which is the limiting step in 
>CO2 acquisition so the thinner it gets, the faster the uptake of CO2.

The research I've read indicates that water flow increases photosynthetic
rates, not aeration per-se.  It's relatively easy to set up a tank with
plenty of water flow to decrease boundary layer thickness without using
aeration.  Dupla recommends a swirl-pattern with a vertical axis down the
midpoint of the tank.  I've done this on two tanks now and get very good
plant growth with very little surface interaction and no aeration.  Plans
for this type of system are on the Krib for viewing.

>Airstones also do not 'drive' off CO2 ... they will equilibrate all the 
>gases in the tank to atmospheric levels, nothing more, nothing less. This 
>idea likely got started by proponents of CO2 injection who couldnt stand 
>to see their CO2 levels drop even slightly ... in fact in most cases CO2 
>injection is unnecessary. 

Since in an aquatic system without a great deal of surface interaction, CO2
levels tend to build up to higher levels than if aeration, waterfalls, etc.
are introduced, it can be argued that aeration does drive off CO2.  The
point of discrepancy lies in what's considered 'normal'.

>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) ... the second is when growing plants which are unable to utilize 
>bicarbonate (about half of aquatic plants have this ability).

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.

If you're running CO2-injection or trying to maintain CO2 levels above
equilibrium, aeration will work against you.

For any newcomers, higher CO2 levels aren't bad for your fish until you get
excessive.  Equilibrium is probably in the 2-4 ppm category.  CO2 levels up
to 15 ppm are quite safe.  Lethal CO2 levels are usually in the 30 ppm
range.  Increasing CO2 levels in the water does not decrease the dissolved
Oxygen levels in the water.  If you have plenty of happy plants and bright
lighting, you can actually supersaturate your water with O2 from your
plants by maintaining a high CO2 level because CO2 acts as a fertilizer in
this case.

I'm currently not using my CO2 injection apparatus on my tanks (don't want
to be pruning every other week right now), but I still maintain low
air-surface interchange rates because my plants seem to do better and
because I have plants that can and will use up my bicarbonates if the CO2
levels drop too far.

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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