you can add other nutrients such as corn meal or straight corn. corn meal is prolly best just like add 1 lb of corn meal per 1 gallon of water, do the same with sugar, maybe a lil more, add about 3-4grams of yeast per gallon. as far as how much, I use one 2 liter coke bottle every 5 or 6 days for three lowrider plants, and I can see a huge diffrence, so i would say prolly as least double? as you can see by my picture its time for me to change mine. another thing is if you heat the mixture to about 125-130F for about 30-60mins it helps break down the nutrients and allows to yeast to eat better...but if you do this wait until the mixture is cooled to room temp b4 you add the yeast or it will die.