Both, I tend to add about 6 drops of chlorox bleach to every gallon in my res since I use R.O. water since the carbon filters remove all of the tapwater's chlorine and 6 drops basically puts it back up to city water levels. It really helps keep my roots white and disease free. The plants can tolerate quite a bit higher levels without issue, but I like to use as little as possible to reach the desired affect. I used to use peroxide but chlorine has better staying power as the h202 breaks down after only a couple days. I feel that if the plant is not busy fighting off disease then it can put all it's resources into growth and believe it or not chlorine is actually a necessary nutrient naturally occuring in the soil in low levels. It sounds harsh, but I can vouch it only has positive effects for me. My res always smells sparkiling fresh and doesn't have that mildewy smell I notice when I don't add the bleach. Just make sure you use pure chlorine bleach without any other additives or scents.