Yellow-purpleish leaves

We are using an indoor ebb and flow system. everythings been fine up until now. The temp stays constant for the most part between 60-80 at all times.. All of the buds look great and im sure thats all that matters... i've read all kinds of posts about what to do but i want someone to see my pics and tell me what they think....
I am about 3 weeks into flowering and our fan leaves started turning yellow.. (PICS IN GALLERY) we are at 12/12 with a chem level at about 700 and 6ph... we are using tiger bloom (2-8-4) blossom builder... could someone please help with my problem..

If the buds are nearing maturity, this is a perfectly normal reaction to the end of a cannabis plant's life cycle.The plant is using the nutes primarily for flower production and no longer requires leaves, because the leaves won't help the plant reproduce,it's primary niche in this point in the plants life cycle. If the plant is not near maturity then there is either a nutirent def., or heat stress.Post pics for a better diagnosis.

i have current pics on my profile in the album...