I assume you are using the vodka as a carbon source to elevate bacteria growth to out compete algae growth? If so, I have found the right amount of carbon dosing (I use white vinegar) to be when the glass gets a white film instead of the normal brown/green film. For my 34gal reef tank, this is ~12.5ml of white vinegar per gal of top off water (kalkwasser). Based on my evaporation, 1-2ml per day. However, each tank is different. Overdosing carbon can lead to a bacterial bloom (white cloudy water) which is harmless and removed by skimming. There are additional benefits of combining kalkwasser with carbon...but you can google that.