I have always had great success with Vodka dosing. I always used the 100 proof, don't know if that matter so much. I used a BRS 1.1ml doser. Would dose about 1.5ml in morning and another 1.5ml in afternoon. I feed daily, sometimes twice and had a lot of fish. My nitrates stayed around 0 most the time. My question to you would be, what kind of Test Kit are you using, are you sure it's accurate. I always had good success with the Salifert test kits. You might check that out because 15ml is a lot to not be seeing any results from. My tank was a 60 cube with a 20 sump. I have vodka dosed on and off for years and never had 0 results.