PH Level

I'm using a hyro bubbler system that I bought and On the direction It said to have your ph at about 6.2 to 6.8 I was wondering is to high I read a lot of post that are mentioning 5.5 I have been running mine at about 6.6 and the leaves are not as dark green as I would think they should be they are more of Lighter green could this be my problem. They also don't have the Growth rate that alot of you guys are getting. Any Advice would be greatly appriecated. Bizzy

i use flood n drain system but because i use rockwool i have mine on 5.5,but otherwise i believe its about 6.2