WOW,
I have never see the top leaves a plant turn so yellow, whle the rest of the plant stays green.. That is BEAUTIFUL but not good..
It is definitely a nutrient issue... it looks like you even have some nutrient burn on all your leaves (brown tips).. BUT the yellowing of the top leaves is a deficiency...
read this page
Cannabis Nutrient and Deficiency Table
NO NUTRIENT PROBLEM CAN BE SOLVED UNTIL THE PH IS CORRECT..
PH LEVELS DETERMINE A PLANTS ABILITY TO FEED IT SELF (ABSORB NUTRIENTS)
keep us posted...
lovelovelove
IF it were my plant, I would give the plant a thorough watering with 5.5 pH water..
cheers