Between 5 & 10, Easiest way is to hold the Vile up to a blank piece of printer paper. Also keep in mind the Nitrate test works a bit differently with API Shake bottle 1 for 30Sec add 10 drops, cap the vile invert for 15-20 sec, add the next 10 drops after shaking 30 sec and invert the vile for 60 secs then let and sit for 5 minutes.