How To Remove Green filmy algae on surface

OK, stupid observation of the week.  On several of my tanks over the last
few years, I've had an annoying buildup of a green surface algae that
blocks a lot of light and has caused me a lot of hassle skimming it off at
water changes.  Turns out it only happens in my tanks that use powerheads
(and no airstones).  Found out by accident last week when I stuck in an
air-driven sponge filter I was seeding, into a tank that had a
powerhead-driven sponge filter, and returned a week later to
completely-clear surface.

Anyway, thought I'd pass this on, since there are probably others on this
list like me that avoid airstones in some planted tanks.  (As of today, I
only avoid them in CO2-injected tanks and places where I can't deal with
the noise.  I have a skimmer on my big planted tank, so this problem never
shows up there anyway.) 

   - Erik
   in unseasonably-sunny Seattle