Cant seen to combat my nitrite

i have that kit.... it had ammonia nitrite and nitrate as well as ph (high low).....the kit being 2 years old.. is that too old????

API test kit is the best out there in my opinion.

check the Lot No. on the back - last four digits are month & year and it needs to be within 3 years of that to be considered in date.
 
i was testing my water again today....my ammonia went from 0 to <.25ppm, nitrite went from .25ppm to <.25ppm (between 0 and .25) and my nitrate went from ~20 ppm to 0..... is this common.... should i be worried????

also i have been doing 25% water changes twice a day (cleaning with the gravel vac to get all the excess waste out of the tank)
 
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