As a side note I've had tanks that continually tested 0ppm for nitrates
but had enormous amounts of plant growth. They were heavily populated
south american cichlid tanks with about three inches of hornwort
covering the top of the tank. Ammonia got eaten up as soon as it was
produced and those guys produced a lot of it. So I don't expect there
is a perfect number for nitrates out there - I always keep adding
plants until it reads zero consistently and I've never had a problem
with nitrogen defficiency.

-Daniel