Surface Film and DIY CO2 (yeast)

First, to Mark McClean:

>     I am new to planted tanks and am having a problem with my 125 gallon
>setup.  We are using vermiculite and soil under a gravel substrate.  The
>tank has been planted for two weeks now.  We are growing plants that do well
>in low light,  We have 160 watts on 11 hours a day.  Everything is growing
>fine but every day a slick clear film forms all over the surface of the
>tank.  The pH is 6.8 and the water is soft.  I have been skimming it off,
>but every day it returns.  Is this related to having plants ?  I have never
>had this problem in any other tank and at a loss as to what to do.  I would
>appreciate any help.

I noticed the same thing in my 29 gallon tank.  A few days ago I added two
black Mollies and the film dissappeared.  I also added a solution of K2SO4
and water (a supplement to my Flourish nutrient mix based on the PMDD
recipe).  I suspect the Mollies are responsible for clearing the surface,
since I see them "skimming" the surface as if they are eating something.  I
don't think they are gasping for air, because they aren't breathing hard -
they really appear to be grazing at the surface.

Now, in reply to Douglas Duncan about yeast CO2 mixtures:

>I'm running one bottle on a 75g and one on a 50g. The recipie I got from a FAQ
>file off of the AquaLink web site calls for 3 cups sugar and two tablespoons
>yeast (roughly 1 packet) to the two litre bottle. The file states that this is
>supposed to last about two weeks. I'm barely getting one week on mine.
>Can anyone give me their recipies, tips on stretching out the time between
>refills, etc.. My wife says to add more sugar to the bottle. I also
>thought that
>placing the bottle on a heating pad might increase c02 output, but then again,
>that might just make it burn up faster.

I mix my concoction in a 3 liter coke bottle.  First I dissolve 1 1/3 cups
of sugar into 8 cups of water (1/3 cup sugar into 2 cups water, 4 times).
Then I dissolve 1/4 tsp. yeast (NOTE:  this is MUCH less than what you are
using) in 1 cup of water and add that to the bottle.  Final contents:  9
cups H2O, 1 1/3 cups sugar, 1/4 tsp. yeast.  If you want to produce CO2
more rapidly, add more yeast.  If you do add more yeast, it will use up the
available sugar faster, and shorten the life of the mixture.

I use this recipe on my heavily planted 29 gallon tank, and it will last
for a month or more.  It produces CO2 very slowly, but I have designed a
very efficient DIY CO2 dissolution tower and my tank water stays at pH
6.4-6.6 (KH = 2deg -> CO2 = 15-20 mg/L).  I think the important thing to
consider is not how much CO2 you are producing but how efficiently it is
getting dissolved into your water.  Also, I do not use a heating pad or any
other warming device.  The bottle sits inside my cabinet at room temp.

Also, I have heard that some people add Baking Soda to their yeast
solutions.  This apparently buffers the pH of the solution so that the
yeasties stay alive longer.  I'll let others expound on this method.

Good Luck,

Steve in Ann Arbor, where a week ago it was 30 deg F with rain/snow mix,
and last weekend we hit 90.  Go figure.