I have a 100% hydration 50/50 whole wheat and white flour starter. A while back, I was having trouble maintaining its health. No matter how faithfully I'd stick to feeding schedules, it would develop a weird rotten egg smell. I believe that this was due to discarding too much of the starter every time I fed it. The acidity that was making the starter "immune" to colonization by unwanted bacteria was being diluted too much.