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Test kits

I have noticed there are many folks who have problems with their test kits, 
recent posts have discussed the importance of using fresh kits.  I would 
like to add that it is important to follow the instructions EXACTLY as 
written, especially in multi step tests.  I would like to recommend a few
simple things that most aquarists can do at home to determine if their test
kits are working.

1. Measure distilled water.  This should give a zero reading with virtually 
every test except pH.  If it doesn't the kit is suspect.
2. Run a standard sample (known level).  If it's a chemical you are adding 
you can probably make this yourself.  The sample is best made with water 
that contains all the other chemicals your tank water has and is the same 
pH.  If your standard is made with water that closely resembles 
your aquarium water, you will detect erroneous results that you might miss 
with a standard made in pure water. A standard made with distilled water is 
better than nothing.
3. "Dilution recovery or add back"  This is where you measure your sample, 
then dilute it a known amount with distilled water, and measure it again. 
 A variation is the addition of a known amount of chemical to your sample 
and remeasurement.  The idea is if the test is performing properly, it 
should respond in a predictable way to dilution or the addition of more 
Say you measure your nitrate and it reads 5 ppm.  Mix one part aquarium 
water and one part distilled water.  Your nitrate should now measure 2.5 
ppm in the diluted sample, right?  Try adding one part aquarium water (5ppm)
one part of a 15 ppm solution that you make (assuming you have access to 
the chemicals and measuring equipment to do it) and measure the mixture. 
 It should read 10 ppm, the average of the two.  I tried four different 
nitrate tests and only one passed this simple test!

Most tests probably work fine in pure water with their respective analyte. 
 But real aquarium water (especially fertilized plant aquaria), often gives 
erroneous answers.  I try to stick with a particular kit for each analyte 
and don't get overly worried about the EXACT value.  I usually only care if 
that value is rising or falling compared to the normal level for that tank. 
 You can rapidly drive yourself nuts trying to maintain 0.1 ppm iron with 
your PMDD using three different kits that all give different values.

Hope this helps