I'd suggest checking your test kit, having your lfs
confirm your test or just retesting yourself again. If your tap really have 20ppm of nitrate and you just finished cycling (and haven't done a water change) then it's highly unlikely that your nitrates are 20. Your cycle would have been creating nitrates in massive amounts. Adding those to the existing 20 that is in the tap water doesn't equal 20.
I live in the Philly area too. My municipality reports nitrates out at approx 1.5ppm, my tests confirm them to be very low as well. Though those levels can change over the course of a year.
If it really is 20ppm then you need to contact the EPA. I believe the highest level allowed before drinking water is considered contaminated (called MCL I think) is 10ppm.
For the record, I use RO
in my SW
and went back to tap in the FW