API tests are fine, but please understand that these tests are not perfect. Snippet from a person working for API:
"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 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."
Logically, if the plants were not doing well before the drop in nitrate reading, does it seem more likely that they would be the cause of your nitrates going from 20-40 to 0 in a two week period, or your test solution separating?
So lets focus more on some of the variables that could be effecting your plants by comparing what is different from your previous set-ups and the new.
- What size was your old tank? (Height matters most in this regard in comparison to lighting)
- What kind of lighting did you have on your previous set-ups?
- Are you using the same kinds of plants or different ones?
- Are you living in a new area with different water?
- Are you treating the substrate differently? (In regards to cleaning it)
- How does your stocking level compare to your previous tank?
Plus anything else you can think of.