Sick, possibly dead plants! please help!

I have 19 plants growing in the versagrow 10 pot system(2 of them), under 400w MH running @ 300w in a EBallast, feeding currently with 200ppm of AN Sensi Grow and B-52, added to water that is half ro, half tap, cal-magged to 180ppm. They are watered 4 times a day for 15 minutes, the other 16 plants in the systems are doing excellent with same nutrients and same feeding schedule, even plants that are younger by 2 weeks, so I assume it's not nutrient burn!

I did have the humidity extremely low in my tent, an important note(about 21-30% humidity at 75F). Also, I washed off my airstones with dishsoap when I did my most recent res change and clean and may have had some dishsoap residue that got in the res, figured it wasn't a HUGE deal, cause it's not killing the other plants. Ph also managed to raise itself up to 7.0 which i did not notice for a day or two, so another possible variable in the issue, I managed to get the PH under control now at about 5.8-6.2. It seems the new/top growth in these is starting to have the issue, and it's only happening to about 2-3 of the plants in there, but the leaves are crispy to the touch and dead feeling, started with only the lower growth but seems to be reaching the top growth. Also had a powdery mildew problem which I treated with a spray on organicide, it was however only on the surface of the rockwool and seems to have since been erradicated. Just to note this is my first hydro venture, have done a successful soil and co-co grow consecutively. Yes I know, lot's of issues so far! However all the other plants are growing at an exponential rate! Anyone have any ideas if these can be recovered/what would be causing this issue? Thanks for any help guys!

Another potentially important point i should add is that the light is about 1 & 1/2 feet away, in cool tubes from the plants, and the lights are going 24/7

Anyone have any idea whats going on with these ?

still noone ? there's like 300 views and noone knows what's caused this ?

wow. From looking at them id say it was something you sprayed on them.
If you spray your plants daily with a mix of water and 3 ml a gallon of H2O2 it will kill any mold bacteria before it ever gets started.If you give you plants silica it will prevent mold from ever starting.
normally mold will not grow at that humidity level though.You are way to dry.
stay away from dish soap clean with H2O2 This will clean and sterilize without hurting your plants.
If you are running a 1000 or 600 watt and this plant is in veg back em up off of the smaller plants.
Things you can do to raise humidity.spray your plants with water frequently. add a humidifier, raise temps some warmer air will hold more humidity.even setting open pans of water around would help.
I doubt if a ph of 7 hurt the plants at all.
I hope this helps.Remember water droplets can if the light is to close work as a magnifying glass and burn the leaves of the plants.Any bad leaves id try throwing away.
Hope this helps.

Critical Stage ! Watering this plant for a week or two will give you the answer ! Don't add any thing else and get this plant away from the others just in case ! Good Luck !!

Try to eliminate the bad leaves or at least the sick parts of the leaves

I didnt spray anything on them, only water, once this started, i did have an issue with powdery mildew on the rockwool a bit ago, and I exterminated it with some organicide, seems to be gone now, I picked off all the messed up leaves and kept them in with the others, added a bit of h202 and cleaned/scrubbed my entire system, seems to have cleared up.