yellowing oin middle at top but only middle please help newbie

soil from garden in 10 litre but i added 4 inch of coco an pebble at the start of flower
600hps
1 fan over the tops only to cool light
40- 60 rh
ph 6.8 or below never over an runoff 5.9

bio grow for veg
biobloom now for flower
cannazym
an rhiz in veg
temps form 72 to 84 day goes down to 67 by the morn 80rh


my plant is 6 weeks into flower first it showed signs of over fert so i flushed an calmed the feed down but i give it more water now but last two days ive noticed the cause i fimed in an has four colas in the middle the leaves have turned yellowish but not around the out side whast could this be or is it natural but i dont no why the browns there i have the fan blowing ovver top but at night the rh goes up to 80 by the morning other wise when lights on is 40 -60 please help all advice welcome

Take a pic without the hps on

pseudobotanist said:

Take a pic without the hps on

Click to expand...

could it be nute burn cause i flushed it but now put some nutes in thinking its k probs i dunno ill post tmoz now cause its sleep now