My pH is ok, and my nutrients is appropriate. The roots of your plants may not stay really really white depending on the type of nutrient you use and what color it is. In situations where root rot appears it is usually due to water with not enough oxygen and water that is too warm.
Warm water is a haven for fungus and bacteria.
Keep an eye on the temperature of your solution and don't let it get above 70 degrees. This will help keep the Pythium and other root rot at bay.