Lack of oxygen in the root zone. Make sure your water is about 20 degrees Celsius. 3% hydrogen peroxide in water mix will add oxygen to roots. Adding an air stone to feed air to roots constantly is probably the best idea. I hope this helps, good luck and happy growing
are the roots slimey or just brown. Some Nutrient can stain your roots brown. if it is slimey then it can be caused by lack of bubbles. Water temperature above 70f can caise it. light leaks are the main causes.