Soak it in RO
or distilled water until it thaws and strain through a brine shrimp net (the super fine white ones)
If you don't have an RO
system, get some Home Depot 5g buckets & lids, rinse them out, and fill them up at a Culligan station at Wal-Mart or another RO
water station at a grocery store (Hy-Vee up here in IA, Publix down south, etc). Call Culligan they'll tell you where they have them in your town.
Also I would beware of a false reading on your test kit. I'd be surprised if your municipal water source has 0 Nitrates. Around here, some have 20-30.
Without running a skimmer, refugium, ATS, or some other means of nutrient export, you're gong to have to up the frequency of your water changes drastically, it's the only way to reduce Nitrates.
How long has the tank been running?
What test kit are you using, and how old is it?