I thought I would jump in and tell you all my success with Vodka Dosing. Like MIKE810, no matter how many times I changed the water, my Nitrate levels wouldn't drop below 20ppm. I spent days researching vodka dosing to ensure that it was not a fad and it would work. Here are my results, using 80% proof Vodka:
first day - 0.5ml skimmer starting to generate black scum
seventh day - 1ml
14th day - 2ml
21st day - 3ml
8th week - 8ml nitrate starts to drop (sorry but I did not record the drop in nitrate levels)
9th week - 4ml, nitrate reading 0 ppm (50% reduction in dosing levels)
10th week - 2 ml etc..
As my results suggest, I did not see a positive change in the Nitrate levels until the eight week, so stick to it.
Mike810 - Is your skimmer working? Why I ask is that my skimmer wasn't working very well - only generating a small amount of skimmer waste. To improve the performance of the skimmer I had to generate additional air bubbles with two supplementary air stones inserted into the skimmer body. This temporary mod keeps my Nitrate level to 0ppm so I do not have to dose anymore.