It's a 20gal. I dosed a 1/4 tsp last night and my nitrates are still the same. I'll wait tonight and dose a 1/2 tsp tomorrow
Do not rely on your test kit to give you accurate results. Adding 1/4 tsp of KNO3 to 20 gallons of water will put the nitrates right around 10 ppm. Which is right where you want to be.
And yes, pick up some KH2PO4 and a trace mix like Plantex CSM+B.
See here http://rota.la