Vodka or ethanol (CH3CH2OH) is a source of carbon that bacteria needs. The beneficial bacteria that we are after consume nitrate and phosphate. Sugar can also provide the needed carbon as well. However, overdosing causes corals to brown up because zooxanthellae can also use the stuff. Oxygen can be depleted in the process since bacteria use O2 to metabolize ethanol (or sugar). So you really need to be careful. Besides, bad bacteria can also multiply using carbon as well.
The dose is increased slowly over a number of weeks. According to a German marine biologist, it goes like this:
You can start with 0.1 mL/100 L for the first three days.
Then you increase the vodka volume to 0.2 mL/100 L for day 4-7.
Subsequently to this initial week you increase the vodka dose by 0.5 mL per total tank volume (this is important, do not dose on a per 100L basis, but on the total tank volume!) every week.
When I reached somewhere around 2.0 ml for my 120 gallon tank, I started slowly losing my yellow striped cardinals which are very sensitive to a low oxygen condition. The loss occurred over a month or so that I did not realize what was happening until I lost 4 or 5 of them. They were thriving and spawning in my tank regularly before vodka addition. When I stopped dosing vodka, I stopped losing them.
My corals remained bright and colorful after I stopped. I am starting to dose very slowly again to experiment since my corals browned a bit after a low salinity accident which resulted from a leaky pump on my calcium reactor.
If you are really interested in vodka dosing, here's the thread where a lot of discussion took place:
http://www.reefcentral.com/forums/showthread.php?s=&threadid=288714&perpage=25&pagenumber=1
Tomoko
. I have stolen from my tank in a pinch, though 
