yellowing leaves in my first grow, please help a brotha

hey, i have 2 female durban poisons and 1 mystery plant that im growing indoors.. they are about 6-7 weeks old and they have been doing fine until now. all of a sudden, all at the same time, they began growing leaves that eventually turn all yellow.. if you look at the pictures, youll see what i mean. . i have been using bottled water and i checked the ph of my soil and it is about 7.8-8.2 and i know this is not good . I recently about 4 days ago, 2 days right after the new growth began yellowing, transferred the plants to happy frog potting soil and the problem seems to persist. anyone have any idea what it could be? or what i can do to fix it? i am about 10 days away from starting the flowering cycle and would like some opinions... one of the plants is worse than the other 2... im using a 400w MH btw and the temperature of the room is 75-80.. thanks
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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