White scum?

Ok -- what's going on now? :(  Suddenly, I have a thin white film on the
top of my tank, kinda like an oil slick. It looks thick and yucky when I
look up at it but when I open the hood and look down on it I almost can't
see it. If I stir it around it breaks up into pieces (like ice floes) but
doesn't dissolve. What is this? Can I get rid of it? Does it have to do
with adding more fertilizer lately?
Tank specs:
48 gallon with biolife filter
23 different kinds of plants and 30 fish
DIY yeast CO2, two bottles running through power head
substrate of gravel and Terralit
Temp 79 F
pH 7
iron .05
KH 20 (2.2 dh)
nitrates 30 ppm

in sunny, warm Vancouver