Yellowing...can someone tell me what im doing wrong?

Im a patient....Im in week 5 of flowering and some of the leaves on the buds have turned yellow and some can be easily plucked...this is what im using
-6- 600w hps
-90 gallon res

-no RO system just chlorine and sediment remover
-Bigfoot A&B bloom
-Root Excelurator
-Big Bud
-Hygrozyme
-Carboload
-SM-90
-Ooze
-Boost Juice and sweet
-temperature between 77-81 while awake...72-25 while they sleep
too many nutes? nutrient deficiency? lack of nitrogen?
I would post pix but they're sleeping
Please help
Thanks

They do turn yellow in late flowering.

It could be a lack of nitrogen.

It could be a few things.