first growing, problems with some plants

Hi!

I'm growing El Nino. One week ago, the leafs of one plant started to get yellow, and now its dry. And now the other plants have the same problem. In the last week the weather was very hot and dry (but i use aricondition and humiditor also), there was one day when they didnt get water. The temperatur, is minimum 22° maximum 34° and the humidity is min 40% and the max 77%.
But I have Ultra Haze and Nexus type of plants also in the same area and they are healthy.
(I was thinking about it could be pH problem or maybe some insect.)

Please help me.

Thx

I'm not a soil grower so can't really say but do have one question.....did those leaves get wet while you were watering and dry in the sun? They look burned.....

Those leaves are in the middle of the plant and some top leaves have the same problem, when i water them i allways do it carefuly, so they dont get wet.
i looked the pH in the soil, and it is 5, but the water what i use is 6.5.
Is it possible that the El Nino is very sensitive for the pH? (the two other type of plant is growing in the same room and they get the same water like El Nino, and they have no problems)
Should i wash through the soil with fresh water?
Thanks a lot

pH is way low- base it up until run-off is @ 6.5

if you check up on the el ninio the ph they prefer is 5.5 so your waters ph is to high not to low