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Nitrogen in a new tank

OK, after months of reading/planning/dreaming about my first
planted tank, I've finally set it up, but now I'm not sure I
exactly understood the advice I've been trying to follow.

As I understood it, planting heavily with fast-growers is
wise in a new tank, to soak up nutrients and starve algae.
A second prong in the algae-starving attack involves not 
feeding fish (or not having fish at all initially?)

So I planted about 2/3 of the tank bottom with fast growers.
I put three 1" otocinclus and three 3/4" SAEs in the 40
gallon tank. I figured the fish:plant ratio was low enough
that I'd be fine without a filter for now, which would help
keep the CO2 levels high. But to make sure, I decided to
watch nitrogen levels, and now I'm starting to think I've
got something wrong here.

The evening I set the tank up, all of the plants were bubbling
away. Nitrates read about 3 ppm (by interpolation on a Tetra
test kit), nitrites read about the same color as "less than .1
ppm" (Tetra kit), and ammonia reads the same color as "zero ppm"
(Tetra again).

The next evening, the plants were bubbling again, somewhat less
vigorously, and all three nitrogen tests give colors farther
downscale than the lowest readings?!? OK, so the tank is
nitrogen limited for now. Fine.

But a bit of thinking made me wonder...won't the tank just
flat-out run out of nitrogen? I'm not feeding the fish or adding
any nitrogen-containing fertilizers, and I can't think of any
place else nitrogen can come from. (Are there aquatic legumes? :-)

So how does the algae-starving plan not doom the plants? I guess
I'm missing something, but I'm not sure what.

Possibly relevant tank parameters:
40 gallons, 60 watts of gro-lux bulb, 60 watts of vitalite bulb,
laterite/vermiculite covered in gravel for the substrate, yeast
CO2 injection, tap water pH is about 7.8, with CO2 it is down to
about 6.1 and probably still dropping (I don't know kH yet; test
kit is in the mail), 75 degrees.