there is a popular formula method for the GH flora series(lucas formula)that advocates a reservoir change after a equal amount of water has been added back,from what i've read it's a very effective method and produces good result's,i havent tried it yet,but i plan to in the near future,i have a 30 gal res that i replemish every 2 week's with added nutes and a 50%water change and my set-up is thriving very well,but since i'm a minimalist it works out well.there well never be a perfect balance of nute uptake,the plant is selective so it will use only what it needs,but it will end up using most of everything.just my suggestion,but get input from others,then decide for yourself,chances are you'll make the right choice's....good luck to you WM. bonbon