I have a book called "Marijuana Buds for Less, Grow 8ozs. of bud for less than $100" By SeeMoreBuds
This guy is actually a member here of RIU.org he goes by the name GardenKnowm. I think he is on vacation for the next couple of days so I'll defer to his book.
For example, if you have a starting soil pH of 5.1 the water pH needs to be 7.4. I'll make a chart:
Soil pH = Water pH
5.1 = 7.4
5.2 = 7.3
5.3 = 7.2
5.4 = 7.1
5.5 = 7.0
5.6 = 6.9
5.7 = 6.8
5.8 = 6.7
5.9 = 6.6
6.0 = 6.5
Etc, etc.
You said your soil is between 6.5-6.8 so that means your water pH, after anything you add(nutrients, molasses, etc.) need to be between 6.0-5.7 when you water. You can raise pH by adding baking soda to the water and you can lower pH by adding vinegar.
Buy SeeMoreBuds book. You can find it on the web or at Barnes and Noble. Its a wonderful guide and a day by day trip of pictures through the life of a plant from seed to harvest.