Well flush the shit out of it then! This is one of the biggest benefits of using rock. Using PH balanced water flush until your ph coming out is the same as going in, this make take 50 gallons but the amount doesn't matter. Once your ph is corrected then back to your nutrient mix...balanced of course and your done. This can all be done at once, in a couple of hours you will have corrected your problem and the girls will never know or skip a beat. Ph every water and feeding, introduce a leech using at least twice the volume of water you usually use, once a week. It will always fluctuate, the key is to keep it stable...JAS