According to a pro brewer on the BA forum stick with simple sugars, use a neutral distiller's yeast at 4 million cells/mL/*P, about 5 times as much yeast nutrient as you'd use in a beer, oxygenate to 10 ppm and after about 6 hours either CO2 pump or continuously rouse. With this method his 1.026 OG seltzer gets down to 0.996 in about 24 hours, at which point I guess you flavor to taste.