help with this algae/ scum on the surface

["Loh Koah Fong" <kfloh at ntu_edu.sg>]

Hi,

Some time back, I noticed some kind of greenish powder on the surface of my 4ft tank. Covering maybe 60% of the surface. No smell. See the picture of the algae at http://www.ntu.edu.sg/home/kfloh/photos/what_algae.jpg

It caused my fish to go to the surface in the morning as if lacking of oxygen. It has wiped out my entire stock of 20+ Harlequin Rasbora and some Cardinals. Before they die they are swimming weakly and haphazardly near the surface. The scum seem to be concentrated at a corner of the tank above my E. bleheri which has some leaves growing out of the water. The water is quite stagnant over there.

The tank was set up 2 years ago and nothing significant was done lately. Currently I am using DIY CO2 which I run it through a reactor non-stop. It has not cause problems before. And I am using a skimmer to remove the scum weekly.

I wonder how it originates and how to remove it?

Thanks for any help.

koah fong
http://www.ntu.edu.sg/home/kfloh/mytank.htm