Fish are cold blooded. Cold blood means the temperature around you governs the speed of your brain, heart, lungs, all bodily functions. With a sudden shift to cold temperatures, with ectothermic crittters, the body's functions slow rapidly, the first to shut down is the conscious brain, the same with warm bloods too, then the lungs, then the heart, and finally, the whole brain (if not frozen completely first from excessive cold).
Therefore, freezing is extremly Humane. The fish falls asleep seconds after the door is shut (allowing minimal water).
Baking soda just seems wrong, taking oxygen away is plain and simple suffocation. As the lungs dont take in oxygen, carbon dioxide accumulates in the blood and gives a burning sensation. Very painful, very sad.
You asked for Humane. Freeze 'em.
*However, i think the vodka idea was nice, ethical with a buzz*