when you want to change, Add a complete fresh water/nutes change out...
I use 2 different values for GH3-part 5ml per gallon of each in early veg...then 8ml per gallon in late veg/early flower...
2nd to 3rd week of flower...(I mix 5 gallons at a time.)
week 2-3f, I use 15ml grow-40ml micro-40ml bloom)
I add 15ml of calmag+ (I use ROwater)
and I use Liquid Karma 40 ml...remember this is a 5 gallon recipe.

In between changes7-10 days, I add rowater by itself, or with 40ml Liquid Karma.

ph at 5.8 until week 2-3f and then ph at 5.4...5.5...5.6..5.7, and back to 5.8 for the last 2-3 weeks. Hope this helps.
Goodluck