yellowing leaves

Hi everyone, Ive been having trouble with my grow. My plants are just under three weeks into veg but one of them started to yellow a week ago and steadily got worse. At first i thought it might have been the PH of the water and soil but have tested them to both be dead on 7. They are under a 400watt hps lamp in a relatively small space but i have managed to get the temp down to a steady 79-80F but was at almost 90 in the first two weeks. I have only fed them once with very weak nutes at 1/4 dose but as of yet has made no difference. Been a long time since my last setup not sure what to try next. any help much appreciated.
Sorry should mention it is westland compost and the two plants are Brainstorm and Big Bang.
 

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