Stupid nitrate testing

I posted this to rec.aquaria too, and I hope you don't mind too much
about seeing this also here.

I decided to test nitrate level in my two tanks a few weeks ago. I had 
been two weeks away, and it was about 3 weeks from last water changes.
My normal routine is about 30% every 10 days, and there is no nitrate
in my tapwater. Usually I'm very lazy at doing any tests, so I can't
remember when I tested nitrate last. My tanks are fully planted (but
low-tech) freshwater communities, 150 and 280 liters - about 40 and 75g -
and the fish population is near to full capacity. pH near 7.0, soft
water, cycled long ago, no dead or sick fish for ages.

I tested one tank: over 160 ppm. Darn! A quick 50% water change and a
new test: still over 100 ppm. Same with the other tank. Fish look
happy anyway, and plants are not dying. At least Hygrophila polysperma
should start getting brown spots with high nitrate level, but there
were none. So I started feeding very little and doing 30% water changes
about every 5 days. After 3-4 changes, nitrate test still showed levels
over 100 ppm, perhaps even over 160 ppm in both tanks.

At this time I started thinking that perhaps there was something wrong
with the test. It is Sera Nitrate-Test NO3, made in Germany. No use before
-date, and I have had it maximum 1.5 years. It has always been very
difficult to use: three reagents, and one of them has to be measured with
a tiny little "spoon", and I can't never be sure if I have added right
amount it with my clumsy hands. This is the most common nitrate test sold
here in Finland, at least only one I've seen until few days ago.

I happened to find another nitrate test by Red sea fish pHarm. Also three
reagents, but much easier to use because they all are fluids and it's 
much easier to count drops than measure some powder. I re-tested my
both tanks: 5 ppm in the bigger, less than 5 ppm in the smaller tank.
Funny. Of course I would like to believe these new numbers better than
the old ones, but now some questions enter my mind:

- any opinions on nitrate tests, and especially on Sera and Red sea

- how long do they last and what does happen when they get old? Exp date
  for Red sea test is Dec 96, I couldn't find any from Sera test.

- how many of you believe that 160 ppm is the right level, and who do
  vote for 5 ppm? :-) My local shops can do water tests, but as far as I
  know they don't carry nitrate tests, only pH, ammonia and nitrite.
  By the way, I tested nitrite from one of the tanks, and it was zero,
  like it should be in a well-established tank.

Liisa Sarakontu                     INTERNET:    lsarakon at hila_hut.fi
Helsinki University of Technology   WWW homepage http://www.hut.fi/~lsarakon/