waterfarm 8 pack ph help??

So this is my first grow and the one thing i seem to be having issues with is my ph/ppms. Im running a waterfarm 8 pack and my ph in all the buckets seem to like sitting at 6.7-7.0 ph and ive already got 7 babies in the buckets. I tried adding SMALL incriments of ph down but it doesnt bring down the ph in the buckets... help??

Im seriously in desperate need of some help. I just need to know how to bring the ph down without fucking the whole system up..