Algae re-post

I posted this simular message the other day and have not had any response so
here it goes again: Has anyone had experiences af a film of (initially
appears white in its early stages) lime green algae accross the water
surface? There is not one drop of algae in the water column (well, a little
for the ottos). Ive tested Nitrates and they are still undetectable, could
this be the problem? Mind you these have been the conditions for some time.
Iron levels are around .01 mg/l (is that the measure?). I had paused in my
fertiliser (varied it actually by only using Dupla daily drops) for a week
or so when this started to occur. I have since returned to my normal
fertilising regime but the algae is still persisting. I am sick of siphoning
the stuff away all the time. Why would the algae only grow on the water
surface? Any suggestions greatly appreciated.