A quote from the "Grower's Bible"...
"Irrigate soil and soiless mixes when they are dry one-half inch below the surface."
I hope this may help you. Over/under watering can cause a host of problems, and it looks like you have one.
If you have root rot, the roots are actually decomposing naturaly through fungus and mold growing due to the moisture levels in the soil. Causing the important microscopic root hairs to be devowered by the rot, and therefor not able to consume water.
If underwatering the roots simply dry out and die.
Tip from Laylow:
If you water your plant and the water beads on top of the soil, it might be because you aint watering enough. If you have knats and algea on top of soil, might be a bit too wet, a bit to long.