How often do you change your water? I change mine once a week. I had problems with burning before, so I talked with the guy at the hydro store. Some of the nutes settle out in the water (I'm sure you have noticed that already), some of the solids stick to the roots also. That's why when the plants have been growing for awhile the roots won't have that bright white color like they do if you use GH or other synthetic nutes. He advised me to flush the plants with plain water every time I change the res. Once I started doing that there were no more problems. I take it a step further. When I have to add water between res changes, I pour it through the plants instead of just adding it straight to the res. My roots are still mostly a light brown all the time, except for new growth, but no nute burn. How many plants do you have growing in the DWC? I have an ebb-n-flow setup for veg that uses a 5 gallon res, with 10 plants in it.