PH sounds good but back to the nutes. When you mix nutes up are you just figuring the 1 gallon your using to mix or are you mixing strength for the water in rez?? I give example for ya so we on same sheet of music. I believe we have the same system as I have a 55 gal rez too and also use GH. At your stage I would have 27 gals in my rez. At the mix I go by that would be:
Micro 5 ml per gal = total 135ml
Grow 5ml per gal = total 135ml
Bloom 5 ml per gal = total 135ml
florishous plus 8ml per 5 gal = total 40ml
Not saying your mixing wrong just seems real lite for some reason. are you running nutes full strength or partial atm?