I have been using GH 3-part for about 1 1/2 years now and dont have much of a ph flux at all. I do weekly rez changes. during the week according to my meter I have a swing from about 5.2 to 5.9 and that is through entire grow. also when doing topoffs I do things just a lil different I guess. most what I read is peeps do plain ph'd water topoffs in between rez changes, what I do is alternate nuted water and ph water during topoffs. In other words I drain and fill my rez on Sunday. On Monday if it needs a gallon of water it gets plain PH'd water. If on Tuesday it needs a gallon of water it gets a gallon of full str nuted water and just keep swapping back and forth. Now I'm not saying what I do is the best or most correct thing to do but it works really well here!!