I'm not familiar with using the GH series--use botanicare--but my hydro store owner told me to use 5ml per gallon (1tsp) per gallon H202--the 3% store bought kind. I know they make a stronger version but I have not tried it. It keeps out the mold, algae etc and gives the plants a good kick of O2. I add it every third day as it only lasts for 48 hours. I know there are some products you shouldn't use with H202--don't know what they are but this system has always worked great for me.

ps...nice photo of roots.