did you thoroughly flush the coco ? i would flush again. once the ph of the coco has stabilised if you find it is moving towards one end of the scale during feed cycles then adjust your mixture to compensate , ie if runoff is 5 , put mixture higher , 6.3 . i generally find its the other way , ph goes up during the feed cycle , so I feed with 5.8 ish and end up with 6.1-6.3 coco is pretty tolerant though , so if you got big problems there's usually a cause . 1st job flush , flush , flush till input and output of the coco is stable . then re-feed