You could be leaving residue in the tubes that's resulting in false readings.
What are your complete water test results?
Have you kept an active log/journal for all your test results?
The night I decided to start the sorority, my nitrites dropped from 1.0ppm - .50ppm then .25ppm.
Since the initial introduction, I haven't even done any water test or done any water changes.
These are the latest results.
Nitrite = 0 ppm
Ammonia = .25-.50 ppm
PH = 7.2
BTW, what kind of container are you using to do your water changes & how much water does it hold?
Oh yeah, if you're that close to a Petco, bring a bottle of your tank water & have them do a test for you if you're that worried.