please help really important 5 months in and plants dying

[mat369]

iv been growin my babies since march so they are prety far into it(preflowers are showin and should b buddin in few weeks)... im doing an out door grow and all has gone well till recently.. one of my plants started getting yellow leaves and going crispy i thought it was over fert so i stopped, and havnt done it to any of my plants in about a month now. it kept getting worst and is now very close to death! i know im not over watering because i live in spain and its very warm here and im watering them daily so it not under watering... anyway the rest of my plants have started getting yellow leaves (quite alot) including my biggest meter n a half. so now im worried they might die off too. i bought some fert with 10percent nitrogen in it and used that with the water 2 days ago correctly so im hoping it may improve them but i dont know? iv checked the ph of the water with a pool ph tester and it showed up as 7.6 which i know is way off what it should be.. is this the reason for the yellow leaves? and how can i correct it to the rite ph.. can anyone tell me what to add to the water and how much , say to the littre, i will need to add? also i was wonering as i live in spain and the temp is very high is there a possibilty the sun could b too much for them and thats the cause of the dying off?
they are all mostly in the ground(i have 8 btw) so theres no way i could really move them anywhere else, there in a place where they get constant sunlight all day also..

so please can anyone help me sove this problem and save my girls from dying?
chears
matt

[jahconquering]

water with PH corrected water.. 6.2-6.6 is prime time..