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Prandtl boundary and CO2

>Jeff and I have been wondering about how much water movement is
>necessary to improve the CO2 diffusion rate.  Will gentle movement from
>say, a typical canister filter return, provide sufficient movement?
>Would we be likely to benefit from a powerhead "blowing" water in the
>lower portion of the tank?  "100,000 times slower" diffusion rate in
>water sounds like we're going to need a mighty force indeed!<g>  Any
>practical advice here?

Basically, you want to decrease the Prandtl boundary with respect to your
plant leaves without contributing to CO2 loss. 

There are a few ways to do this:

Set up a water flow pattern that moves water past your plants without
recirculating it to the surface frequently (a swirl works well, with the
pump outlet(s) close to substrate level).

Tightly cover your tank to prevent CO2 loss from the air space above your
tank.  Note:  This method can be hazardous to your fish if you have a
sudden need to vent CO2 (reference Dave Gomberg's accident with a funky
regulator).

Increase your CO2 input and dissolution using a reactor.

These aren't the only ways, but they each work.

--
David W. Webb           Texas Instruments
(972) 575-3443 (voice)  http://www.dallas.net/~dwebb
(214) 581-2380 (pager)  2145812380 at alphapage_airtouch.com