Film on water surface

[Jeff Coart <rapdfire at pacbell_net>]

For months I have had a thin film that forms on the water surface of my
60 g. tank within a few days after a water change. It's not algae -
looks more like a slightly oily, whitish slick that breaks up when
disturbed.

Over the last couple of weeks I have increased my trace additions (I
think I have been underdosing based on Tom Barr's recommendations I have
seen on several posts); I also began dosing MgSO4 at 1/4 tsp per week
during this time.  Suddenly, my oil slick is gone. Coincidence? I've
also noticed that my tank just looks better overall - water has a
sparkling clarity and algae that forms on the glass has decreased in
rate of formation.

Is the disappearance of the oil slick due to the nutrient adjustments or
is/was the cause unrelated to this?