i too use dwc for all my grows...
your plant is considered over watered, meaning there is not enough oxygen getting to the roots. you need to either add more airstones or add a more oxygen pumping system- depending on your setup... shoot for 28 liters of air for every 1 gallon (3.79 liters) of working water. submerging netpot into water is okay, but you need the oxygen. typically 15 liters of oxygen is okay, but your plants are drowning...
...i hope this helps...