Hi Dave,

One thing I forgot to mention about the Tetra test kit is that you must
always follow the instructions which tell you to compare the colour when
holding the test tube *against* the colour chart.

If you hold the test tube even 1 inch from the chart you will see a paler
colour which will mislead you.

Many folks (myself included) expect a certain result and will move the test
tube back and forward until it matches their expectations. If you do that
the test is invalid.

I've just diluted a test sample from my stock 20ppm to 5 ppm and again I can
confirm that the test kit is accurate at 5ppm.

Tom must have used an out of date test kit, as I cannot find anything wrong
with the results from my Tetra Nitrate Test kit.

Graham.

BTW I have no association with the manufacturer - i just use their Phosphate
and Nitrate Tests because they are nice and cheap.