While searching the internet I came across this very informative thread on another forum. I know many use the API kits so I am copying it here. For the complete thread you can visithttp://www.fishlore.com/fishforum/test-kits/6439-important-information-api-freshwater-master-test-kit-users.html
Here is the post
Hello all. I have gone back & forth via email with API today, and they've given me some valuable information on the API Freshwater Master Test Kits. The original topic thread on concerns that I had about the test kit results are in a previous topic:
Basically, I was seeing nitrate test results yesterday that I found unbelievably low - zero - in an established tank.
I was very impressed with the quick responses I got from API, and my faith is restored in their test kits. I'll paste in the information from emails with API. Note - I was finally able to get a nitrate reading of 40 ppm after following their directions:
"Thank you very much for contacting us regarding our products. All of
your comments and questions are valuable. We use your feedback to
create the most effective line of aquarium and pond products available.
Your solutions are not expired. Each reagent bottle has a Lot # printed
on the bottle. The last four digits are the month and year of
manufacture. Example: Lot # 28A0102. This is a pH reagent manufactured
in January of 2002. Pond Care Wide Range pH, Ammonia, High Range pH,
Nitrate, Phosphate, Copper, Calcium and GH all last for three years.
Nitrite and KH will last for four years. Freshwater pH(low range) and
Pond Care Salt Level will last for five years. I would not trust these
kits after they have expired.
Liquid Nitrate Test Kits from any manufacturer can have a common problem
with their last test solution. For some companies, it is bottle number
3, but for us it is bottle number 2. One of the ingredients wants to
solidify out of liquid solution. If the test bottle sits for any period
of time, this can happen. If this does happen and the test is performed
without Bottle # 2 mixed properly, then you can get a falsely low
reading. I have never heard of falsely high readings with Nitrate Kits.
I would try tapping your Bottle # 2 a few times on a table or counter
top. This should loosen-up anything that has solidified. Then I would
shake this bottle for about 2-3 minutes, to really mix it up. Try the
test again and hopefully this will fix your problem. With regular weekly
usage, this bottle should only need to be shaken for 30-60 seconds.
Always check results at the recommended duration of time. Letting the
tubes sit longer can result in falsely high levels."
I then asked if heating the nitrate test solution #2 would help get the material back into solution and got this reply:
"I don't think heating is necessary, plus I don't want to degrade the
chemicals in any way. Tapping your Bottle # 2 a few times on a table or
counter top and then shaking should be adequate, but you will
immediately know with the next test."
I then asked if the proper way to match the colors was to place the test tube directly on the card, or back it up a bit, since the color can get a little darker when placed directly on the card. Since the gentleman was being so honest with me, in admitting that there could be a problem with their test solutions (or anyone elses) I thought it would be courteous to ask if I could share this information in the forums I read:
"You do touch the test tube to the color card.
It will be ok for you to share the Nitrtae Test Information. It is a
problem that every manufacturer has and we are interested in the kit
working properly for the consumer. "
As I said, the nitrate test finally gave me an expected reading, and my faith is now restored in the test kits. And I am really impressed with any manufacturer that replies quickly, tells you that there could be a problem with their product, and tells you how to fix it!