temps are high 86 between 12-3 really hot days.. but i normally can keep it to about 80-84 during the hot part of the day. then it normally sits at about 76 after 3 clock. other than that the plants look great its mainly that one plant and then a few leaves on two others. i moved my light higher just to see if its that dunno yet have to wait a day to see any change. i thought it might be nitrogen toxicity so i flushed them.. but now i dont know..
Good that you flushed and raising light, looks like you might have both issues. I'd stick with straight RO water for now until you see new healthy growth and then ease back in with 1/4 strength nutes on a feed, water, water, water, feed schedule.