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Nitrates and testing (question)



I seem to have a bit of an anomaly as far as my Hach nitrate test kit is
concerned.  I have been adding KNO3 (in the form on saltpeter) at the rate
of  tsp. for the last 2 days.  I calculated that the concentration - if the
plants used none - should be at about 6ppm.  However, after the first
addition the Hach test kit shows concentration as .25 Nitrate nitrogen = 1.1
PPM Nitrate.
After the second dose the test showed .35 = 1.5 PPM Nitrate in other words I
appear to be missing 4.5 PPM NO3.  Previously to the KNO3 addition the kit
read zero nitrates.

This leaves me with puzzlement and also a dilemma; should I add more KNO3 or
assume that there is enough based on the calculations.  I guess there are 5
possible reasons for the discrepancy:

The test kit is not accurate (or could be I am not performing the test
properly)
The calculations are incorrect - I will put my assumptions later
The plants are eating most of the NO3 / the green water algae have eaten
most of the NO3
The fluorite substrate has somehow sequestered the NO3 (wild speculation).
Saltpeter is not just KNO3

Here are my general tank parameters
75 gallon
192W
GH - 6
Kh - 3
pH 6.8
CO2 - yeast reactor
Lots of plants

The test kit is not accurate (or could be I am not performing the test
properly) -
Using the Hach test kit is somewhat involved, and includes emptying on
treated solution into another test tube while leaving undisolved crystals in
the first tube.  I am not always confident that I succeed in this.  Does
anyone think that this may significat affect the results?  Also the kit is
about 18 months old.

Calculation Assumptions
61% of KNO3 is NO3
A tsp. of KNO3 weighs 4.86g (I spooned out what the label claimed was 113g
of KNO3 - and got 23.25 spoonfuls)
Actual water column = 65 gallons (75 gallon tank)
 tsp. added at 2pm and again at 8 PM
Tests performed at 6pm, and 1pm next day.

The plants are eating most of the NO3 / the green water algae have eaten
most of the NO3 -
This is an interesting possibility - do plants actually gorge like this.
The plants are now looking a little bit greener.  I also noticed that the
iron level plummeted from 7ppm to zero after the additions of KNO3.  The
algae are also doing better now too.  What was a slight water clouding is
now becoming foggier and greener.

The fluorite substrate has somehow sequestered the NO3 (wild speculation). -
Could this be right?

Saltpeter is not just KNO3
Brite-life Saltpeter Potassium Nitrate - purified granular - for technical
use only.  Sounds like it is just KNO3.  But if anyone can confirm it, that
would be great.

I decided to add the KNO3 due to what I felt was a nitrate deficiency -
amazon swords leaves growing well but not stems, slow light colored growth,
older leaves with holes, stargrass going black and drying out, lower part of
various plants stems rotting away.  I also have a green water problem (I
didn't follow my own advice and put daphnia in the tank before the fish went
in.  The fluorite system seems to make no provision for nitrates in its
chemical additions.  I presume that in many tanks the fish waste provides
adequate N. However, in a high light tank with few fish I think there is may
be a need.  Especially in a new tank where the lights are brighter than they
ever will be again (due to newness).

Well, I will hold off on additional KNO3 additions till I test again in a
day or so, or till I hear from one of you good folks.

David Brooks