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Green film algae

I am a newcomer to the aquarium hobby, and about 4-5 months ago
I set up tank.  I jumped in sort of head first, so although I
have now read all the FAQs, I didn't exactly follow their advice
when I was setting up the tank.  Here are the particulars:

Tank:  70 gal. tall (36" wide X 30" tall)
Filtration:  Fluval canister
Lighting:  4 X 30 watt fluorescent, on 12 hr timer
Substrate:  Coarse gravel
Water changes:  20% every two weeks
Fertilization:  Kent freshwater plant supplement with water changes
Fish load:  ~25 smallish fish, fed heavily
Chemistry:  pH 6.8, GH ~2, KH 1.7
            ammonia, nitrite, nitrate all unmeasurable (?!)
Plants:  Heavily planted with crypts (wendtii?), vallisneria,
         anacharis, hornwort,  echinodorus, hygrophila
         difformis, java fern, duckweed

Anyway, I have a serious algae problem.  The hair and black brush
algaes are unsightly, but what really bothers me is the green 
film algae.  After a thorough cleaning I get a noticable haze after
24 hrs.  If I go a week without cleaning the glass I have a solid
opaque sheet of bright green.  The SAEs pretend to eat it all day
long, and I can occasionally see snail tracks, but they can't
possibly keep up.  In addition, many older leaves (crypts, echinodorus,
anubias) are covered with a dirty-looking film of dark green/reddish
algae that can be scraped off with a fingernail.  Finally if I don't
have significant surface agitation I get a pale greenish "slick" on
the surface of the water.  No green water, just a thin film.

I am trying to get a CO2 system set up, but I haven't yet located
a suitable needle valve.  I am hoping that once I add CO2, the
improved plant growth will help to inhibit the algae.  The
unmeasurable nitrates make me think cyanobacteria, but the stuff
on the walls is bright green.  Any thoughts?

I don't suppose there is any market out there for 3' branching 
stems of anacharis or surplus thickets of hornwort?

David Ozenne
dozenne at 10fold_com