Agreed on the new test kits. If you gonna use them, you might as well get accurate results to work from.
In any case, 40ppm nitrate is nothing to worry over. Just a sign you have nutrients left over for some new plants to thrive from. Try adding some fast growing floaters like salvinia or frogbit.
Truthfully you are not neglecting you filters if they are still providing good water flow. Unless you bleach them, cut the power on them, or overclean the media in them, they will always have beneficial bacteria growing on them. Nitrate is the final product of a working biological filtration system.
The only way to eliminate nitrates naturally other than water changes is anaerobic bacteria living deep withing non-oxygenated soils (occurs naturally not to be concerned with at this time) or finally adding plants that will use the ammonia before the bacteria convert it into nitrate.
So chill, your doing fine you slacker...