Small Green Spot Algae

Can anyone give me some suggestions on how to cope with  the small hard
green-spot algae that develops on the glass in the aquarium?  It also begins
to form on the leaves of the plants and I have had no success reducing the
level of this algae.  No other form of algae can take hold on my tank because
I have 2 sturisomas, 7 SAE, and 6 Ottocinclus affinis.  This stuff, though
has remained for the last 5 weeks no matter how clean I keep the tank, no
matter how often I do water cahnges and no matter how much or little
fertilizer I add to the tank.  I have had the system up and running since
December and only in the past 5 weeks have I been confronted with this
particular variety--bright green round spots about 2mm in diameter--I
wouldn't mind it on the glass, but when it forms on the leaves of my taller
delicate plants (i.e. Cryp Wendtii and Cryp Blassi) it creates dark black
patches on the leaf until the whole top side of the leaf is rough and scabbed
with algae.

My pH is 6.2 (7.0 before CO2), nitrates 0, nitrites very low, phosphates
somewhat high due to moderate fish load, 79'F, regular weekly water changes
(10%) and 80 watts incandescent and 80 watts flourescent lighting on 40
gallon tank on for 12 hours/day.  I have cut the lighting back to 10
hours/day to see if this helps.  The water hardness is somewhat high -- 200
ppm.  But really, none of these water conditions have changed.  The only real
change is that I've added quite a few plants and the tank is rather "packed

What I"m looking for is what general water conditions promote this particular
variety of algae.  I haven't quite been able to ascertain that by digging
through my texts here.  Any suggestions would be greatly appreciated.

Thanks in advance.