maybe serious ph issue

i checked my ph, its at 6.4, ive added a total of 15gals worth of solution to about 4 gallons of water over the past 3 days with no change.... whats the deal here. also ive noticed some white chunky build up at the base of my stem... anything to worry about?

let me refraise because i have a tendency to confuse... ive added close to 10ml, 5 ml is for 10 us gallons.

guessing its nothing serious.