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Re: RE: Nitrate test kits

>I currently use AP (aquarium pharm) nitrate test kit. I am not sure if we
>have the same kit but with the one I have it's easy to tell the difference
>between 0 and 5ppm in good lighting.  I have used the tetra test kit and
>the dry tabs (i.e. crapy tabs) and this one is far superior to them.

Here my 2 cents: I recently run out of a Seachem nitrite/nitrate low range 
test kit and decided to try the Tetra test (high-range). Reasons: half 
the price; and the Seachem test, although giving consistent readings
when used with both the included and DIY reference solutions, wasn't 
very accurate and drifted with time. The color scale was usable only 
if properly calibrated with reference solutions that span the range 
of interest, 0 - 10 ppm. And I found necessary to re-calibrate it a 
few times during its life. So much extra testing decreased the kit's 
life significantly. Could that be an effect of expired reagents ?

To my surprise, the Tetra test is perfectly usable in that range. Its color
scale has 5 patches for 0, 12.5, 25, 50 and 100 ppm. I found quite easy to 
interpolate between the 0 (pure yellow) and 12.5 (orangish) under good
light. Distilled water gives the pure yellow color. The test produces a 
clearly non-yellow color when fed with a 2 ppm NO3 reference, so it's 
capable of measuring the presence of trace amounts of NO3. 5 ppm produces 
a bit darker color, clearly in between the trace response and the 12.5 patch.
10 ppm produces the 12.5 patch color. Well, this is  OK for my needs. I don't
see the point in being able to tell apart say 5 from 7 ppm nitrate, in
particular if that requires a $50 test kit. A scale like: no nitrates (0 ppm),
trace (a few ppm), some (5-10 ppm) and more than 10 ppm has enough 
resolution IMHO. It remains to be seen if the test drifts with time. And if 
it doesn't, I'll stick with it. For $8.50 for 40 readings, it's quite a 

And here is my theory why this test seems to be easier to read than others 
I've seen: most nitrite/nitrate tests use a redish/purplish  color scale 
that uses color *saturation* as the reading parameter. They go from 
white/transparent to pale shades of red/pink, to stronger shades. The 
Tetra test apparently mixes this redish indicator with a yellow dye that 
is neutral to the test reactions, that is, it doesn't change color or 
saturation with the NO3 concentration. In this way the color saturation 
scale gets replaced by an equivalent color *hue* scale that goes from pure
yellow to pure red, with orangish/brownish tinges in between. The human 
visual system is better adapted to tell apart between similar hues than 
beteween similar color saturation levels, hence the easiness in reading. 
Btw, Seachem's phosphate test and TriAqua's pH test use similar hue scales 
and are also quite easy to read.

-Ivo Busko
 Baltimore, MD