Take a sample of tap water and tank water to a pet shop and ask them to test it. Take your test kit too and test the same sample at the same time. Compare the results.
If your test kit reads nitrite but the shop's kit doesn't, then one of the kits is faulty.
If the shop test says there is nitrite in the tap water, contact the water company and the health department and inform them the tap water is contaminated with nitrites. Nitrites cause cancer so if there are nitrites in the tap water, that is a health risk.
If the water company and health department don't care, contact the local newspapers and let them know there are carcinogenic (cancer causing) substances (nitrites) in the drinking water. Let the press do the work for you.
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If you do have nitrites in your tap water, you and your family and pets should drink bottled water until the tap water is made safe.
Have lots of floating plants in the tank and let them use the nitrites. Minimise feeding and don't clean the filter for a month.
A healthy biological filter should break down any nitrites within a few hours.