What's the resolution of your test kit? My math is a bit fuzzy, but that should have raised your tank to around 1ppm of nitrates.
Edit: 10g/1000g = .01 potassium nitrate solution. 100mL of that into 240g at 3.78*240gallons
.01*100/3.78*240*1000 = 1.1*10^-6
Man i have no idea what that all means except it should have raised me up to 1 part per million

