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> I use the normal six cups of water, one of sugar and a teaspoon 
> of yeast in a soda bottle to produce the CO2. To prolong contact with 
> the water, I use an upside down plastic cup hidden behind driftwood. 
> This has a pin hole in the top to let the gas escape if too much
builds up.

This is exactly what I do as well, but my recipe is different.  I use 1
cup of sugar, 3 cups of water, 1/4 tsp of Champagne or bakers yeast, and
1/4 tsp of baking soad to retard yeast growth.  I get over a months
worth of CO2, and if I happen to remember to look at it after three
weeks I can change the water, add more sugar and get another month out
of the same 1/4tsp of yeast.  This works fine for my 10g tank, but for
anything larger I'd consider adding more bottles, started a week or two
later than the first to even out the flow.  I place my bottle on top of
my lights, so when they're on the heat encourages CO2 output, and when
they're off CO2 output is discouraged.

> To agitate the water I have the powerhead section of an old 
> Fluval 2 filter cirlulating water across the CO2/water interface.
> My first question would be to ask if it is a good idea to put this 
> powerhead on the same timer as the lights? That way, when the 
> lights went off and photosynthesis stopped, not as much CO2 
> would be desolved, reducing the pH swing.

The pH swing (about 0.3 in my tanks) is negligible.

> My second question is about lighting. The way I figure it, if I 
> set the lighting timers to six hours on, six hours off, there will 
> be two, smaller pH swings rather than one larger one. Is this 
> correct? Also, will only having a six hour lighting period allow the 
> plants to "get into the swing" of their photosynthesis before going 
> dark again?

I suspect Tom Barr would know what 6-hour cycles would do to your
plants, but again, the pH swing is negligible and there really is no
need to go to that extreme.

michael rubin ~ michael at rubinworld_com ~ www.rubinworld.com