film on water surface

Hi all,
     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.  


In Georgia where it is 90 degrees and too hot to be skimming tanks