surface scum

I'm posting this for a friend...he has a 125 gal that he's had established for
some time with 2 aquaclear externals and UG filters w a (relatively) coarse 
gravel substrate.  He has just started to get the tank 'plant-ready' with the 
addition of VHO lighting and CO2.  Just prior to adding the above, he noticed 
the appearance of a thin surface film, whitish in color.  He's not aware of 
any changes that were made that could be tied to the appearance and the tank 
has been operating (with a fairly heavy fishload and no plants) for about 2 

When he skims the film off so that its a little more concentrated, it tends to
clump. When he looks directly down on the top of the tank it is difficult to 
see the film but its much more noticable looking up from the sidewalls to the 
surface. He has his dH at 4, GH about 5, temp is about 78, pH around 6.9. As 
he's added the CO2 and lighting PH has varied from 6.5 to 6.9 and prior to 
adding these he had a dH of zero...condition has persisted through a variety 
of conditions.  He does not use any fertilizer or substrate conditioning (I'm 
working on him on this :-)  
 I noticed a 
recent post (on phosphate) where someone (Dave?) mentioned that feeding with 
high meat content foods could increase phosphate levels...I know that Gary 
(the friend with this problem) has been feeding a homemade mixture using liver
or beefheart in rotation with other foods for years..can this be a cause?  he 
does 25% water changes weekly.

Any clues as to what this is, how it may have come about and how to get rid
of it?

Jim Morehouse
Lucent Technologies (fondly remembered as ATT Microelectronics)
aljem3 at attme_att.com