Everyones water chemistry can be hugely different. In my own case, using RO filter absolutely makes my water unable to stabalize on it's own. I have to add back sodium bicarbonate to give it buffering capability, even after heavy nutes. Otherwise my PH drops like a stone to 5.0 or so, it's pretty bizarre.

Normal PH Up doesnt cut it for more than a few hours. I have to use baking soda which raises PH, but most importantly, gives the water the ability to stabalize. Water is one area where you have to trial and error on your own, even with volumes of good information on the subject. Use the info. as guidelines, but fine tune on your own.