Black leaf tips in one day - Are they dying?

Hello-

I'm a noobie on my first indoor grow:/ I filled out the help form in the sticky post (below) and hope I've done it correctly. My best guess is they are getting to much nutrient:/

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I started with a "Pre made Hydro Kit" and got miracle grow with it. I didn't know it was miracle grow but after three weeks it just seemed the plants grew so slow and seemed to be dying with drooping, sagging leaves. I had them under flouresant T8 and T12 bulbs. Cool white and warm white.

So after three weeks, I moved on to Foxfarm nutrients (Grow Big & Big Bloom) and put them under a 400watt HPS bulb. Literally overnight the plants sprung back to life and became much bigger, brighter and a more vibrant color green from the top center out. The bulb was probably 20 inches off the tops of the small plants.

Life was great for a few days but it started getting a hot in the room. Hovering between 87 and 90 degrees. This went on for three days or so (currently have an AC unit). Then I started seeing brown spots on the lowest leaves of one of my plants. After two days or so they became much more visible and the leaves became dry and brittleital. After Googleing around I found bugs called Thrips that could be causing this problem. I thought this must be it because I had seen some of these flying around in the room I thought. Anyway bought Safer's plant soap that is suppose to kill them.

But now I have a new problem. It happened in only one day. All of my new leaves growing are turning black on the edges and tips. This is frustrating the hell out of me! It seems to be attacking both plants but more vigorously on the larger planet. The plant's are about 1 month old now. Pictures attached.

Please help!

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/* Help Form from Sticky Post */
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H= I grow hydro - recirculating system
R= I do grow in Rockwool
T= I do grow in HydroTon
D= I have a drip system on a timer. Set to water 3X per day during daylight hours for 15 minutes
E- Indoor
E- hydroponic
E- Rockwool / HydroTon
HCL- Hydro
SCL- Foxfarm nutrients (Grow Big, Big Bloom & Sledgehammer)

E- Filtered Water
E- pH is between 5.5 6.4, kinda hard to tell by using color dropper instead of digital meter.
HRT- PPM is at 790
E- 1month old
E- Foxfarm nutrients (Grow Big, Big Bloom & Sledgehammer)
E- I added Grow Big first and it brought the ppm count up to 720. Then added the Big Bloom which added 60 for a total of 790ppm.
E- 6 T8 TruGrow bulbs. Two of them are warm white.
E- Day between 70-75 degrees / Night Around 65-70 degrees.
HD- Not sure of this but water is cold to the touch.
E- Not using Co2 just an air conditioner and a couple of fans.
E- 18 hours of light. Light on at 4am and off at 10pm.
E- Fans blowing. It's a big area.
TR- Yes I did.

Yes, it does look like nute burn to me too.... have you thought of flushing them? Couldn't hurt. Alos, I don't know about the drip system watering for 15 min 3 times a day. Could be too frequent of watering and not enough. I have found that soaking them good every 3-4 days when they are young and then letting them mostly dry out seems to work well...they don't like their feet to be constantly wet. When they are bigger and still in same pots then I water every other day and only once a week with nutes.

oh wait, you are hydroponic....never mind that last post, I grow in soil mixture. Still looks like nute burn tho.