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Nitrates/green water/test kits (question)

Firstly, thank you to Tom Barr and Paul Sears for their response to my
question.  Here is an update.  I did decide to add more KNO3.  On Paul Sears
' suggestion I made a test solution of KNO3 to test the Hach test kit.  I
used 3 solutions and got confounding results:

Solution = 3 ppm- result = 0.66 ppm.
Solution = 6 ppm- result = 0.88 ppm.
Solution = 12 ppm- result = 4.4 ppm. (off top of scale)

There does not even appear to be an internally consistent linear
relationship, let alone accuracy between the results and the solution.  I
have written to Hach regarding the exact procedure and the effective
lifetime of the reagents.

Yesterday I used a HOT magnum on the tank and cleared the green water.
After, I noticed that the angel fish, all six of them, were exhibiting
strange behavior.  They were all moving listlessly about at the top of the
tank pushing their heads as close the surface as they could and are still
doing this 24 hours later.  After this I tested:

Phosphate (Hach) unmeasureable
Iron (Hach) .02
Nitrate (Hach) > 1ppm
pH - 7.5
Gh - 5-6
Kh -3-4

I am guessing that the outgassing of CO2 due the HOT Magnum caused a pH
change and the angels are in shock.  Does anyone know how long this lasts
and if they will recover (previous pH was about 6.8).  The rest of the fish
seem fine.

I added 1tsp. Baking soda (to get KH in line with GH) and to prevent future
pH shocks
I added ½ tsp. of KNO3 - I was not at this point aware of the shortcomings
of the test results and wanted to push nitrates to 7ppm.
I also added 6ml of flourish iron and the bi-weekly dose of flourish _(2ml)

The plant growth and health has continued to increase since the first KNO3
addition.  The green water is returning - it is now 24 hours after running
the HOT Magnum and the green water is at the misty-hazy water stage but I
imagine in a couple of days it will become completely green.  I have had a
history of green water problems.  In my last tank I after four months I
finally gave up and got rid of the tank.  I tried everything, water changes,
turning off the lights for days, and nutrient control.  Even on that tank I
remember struggling to provide enough NO3 - because the test kit told me
there was very little.

 With the lack of reliance on test results I am unsure how much KNO3 to add
on a regular basis.  If add too much on a daily basis I could reach toxic
levels of 30ppm within 2-3 weeks.  However, if I stand on the side of
caution I could have a nitrate deficiency within days.  In the last week I
have added about 15ppm NO3 to the water column.  I may be able to use the
test kit to determine if NO3 is over 12 ppm but I guess not much else.

My next approach will be to cover the surface with duckweed.  If that doesn'
t work I will get all the fish out, put some kind of netting around the
filter intake and put loads of daphnia in.  Does anyone know if either of
these approaches is effective for green water?  It seems whatever I do it
just comes back.  According the test kit there are undetectable phosphates
in the water column.  If the test kit is correct then what are the algae
living (in fact thriving on).  I don't have any problems with any other type
of algae - apart from some green spot.  Anyone know of any other tricks?.

Anyway Paul Sears asked:
> You are looking at quite low concentrations - is it really possible
> to distinguish the colours well enough to get that accuracy?

The Hach Nitrate test kit comes with a 'color-meter' disc that you hold to
the light and rotate the wheel until you get a color match, I believe you
can distinguish with great precision the color of the solution.  In
addition, the parameters of the test are only from 0-1 ppm Nitrate Nitrogen
(=0 - 4.4 ppm Nitrate).  I guess the precision of the test does not assure
its accuracy, however, I had rather hoped the $45 price tag might.

David Brooks