NO3 disappearing

My question is straightforward, but requires a little background:
1) Nitrate  in tap water at between 10-20 ppm and trace of phosphate,
2) beard algae problem in epic proportions - stabilized for now,
3) 40-gal. tank with 120 watts flourescent light suspended 8" above water
surface (mix of Triton, Vita-Lite and Aurora 50/50)
4) CO2 via gas bottle and Dupla Delta Diffuser (call it an airstone),
5) pH between 7.0 and 7.3 depending on the time of day (CO2 runs only while tank
is lighted, pH creeps up overnight),
6) GH of 110 ppm, unknown KH, although hardness supplemented with CaC03 and
baking soda,
7) water change weekly of 33% in an effort to keep nitrate levels within
reasonable limits
8) Sera 'Florena' fertilizer + Dupla laterite substrate.

I've been waging war on hair/beard algae in my planted tank, and at last seem to
have won the battle, using bleach baths and using a mix of RO + tap water to
make the tank read zero-phosphate and limit nitrate levels while maintaining
hardness and pH of 7.0.  I got frustrated and added Maracin 'CopperSafe' for
freshwater at 1/2 recommended dose, and to my surprise that knocked the algae
for a loop (I saw a note on this list indicating a link between copper in
tapwater and a lack of hair algae).  I still have hair algae in the tank, but it
doesn't like growing on the plants; just the rocks and plumbing, which I keep
cleaning...and cleaning...

The nitrate was my only remaining obstacle, since its present in the tap water,
and always registered around 10-20 ppm.  

Once I got the hair algae beaten back, the plants took off like crazy.  But as
soon as the plants hit the water surface and began to grow above it, the nitrate
level in the tank went to -zip-.  I get no reading on my test kit.  It used to
read between 10-30 ppm in the tank.  So now my tank appears to be

Question: How much nitrate is needed to sustain plant health?  Is this amount
greater given the fact that many plants are now growing above the water surface?
I have many fish in the plant tank and my wife sees to it that they're well fed
(read that as over-fed) so I know there's a source of nitrogen, but is it

My tank is now drop-dead gorgeous, but I don't want to suffer another algae
outbreak because of a nutrient imbalance, or suffer the ominous 'plant-crash'
due to a nitrate deficiency.  Recommendations from the brain-trust on this
mailing list would be greatly appreciated.  Also my thanks for many interesting
discussions while I've lurked the past few months.

Dennis S. Gray
Atascadero, CA (where there are no worthwhile pet supply stores at all...unless
you own a horse :-))