Water kefir needs sugar to ferment. To reduce the amount of sugar still left in your water kefir when ready to drink, leave it to ferment a little longer. Leaving it in the fridge for a week+ before drinking will make it less sweet. It's then up to you to decide your own personal sweet spot for sugar left Vs taste. It also comes down to quality of sugar. I feel better about raw cane sugar or coconut sugar still being present, than I do about white refined sugar.