yeah, i think your prob right about the root rot...i dont personally have DWC, bu i think they run sum peroxide or h2o2 or sumthin to get air to the roots and kill off all the bacteria or wutever...run dat fur a while...a few hours?????
anyways, i think your ph problem may be caused by your small res...how many plants total again? 18 or 20z gallons, right? prob not fill to the brim, as well, sooooo..........wit a bigger reservoir, your daily ph flucuation should be negated.