Tips of leaves curling under on 2 plants in healthy garden. PICS...

Hello all,

My garden seems to be very healthy and responding well to the nutes they are taking. However, 2 plants of the 14 have leaves that are curling under at the tips significantly. I can't seem to figure out why only these 2 have a problem. They are roughly 3 weeks into flower. Anybody have any ideas??? Pics below

My setup:
ph 5.8 - 6.2
water temp 68
room temp lights on 75 - 80
room temp lights off 65 - 70
3 3 X 4 floodtrays
2 1000 mh warm
1 1000 hps
rockwol cube in 1 gal smart pots filled with hydroton pebbles
generalhydroponics floragro, floramicro, florabloom, koolbloom, superfloralicious plus, vegan compost tea, botanicare cal mag.

The ones that curl look like this:

The good ones look like this:

Any help would be appreciated. Thx!

how close are those 2 plants to the lights? looks like they could be drying out a little (the curling leaves all seem to be at the top?) are these 2 plants near the direction switch of your oscilating fan? excess wind can make the leaf surfaces dry out faster...correct me if i'm wrong?

the 2 plants are at least 3' from the lights. The healthy ones are just a little less than 2'. The oscillating fan does not blow any air directly at them. But they are near the direction switch. The fan blows at them but above them just before it switches to the opposite side. I just put that fan there 2 days ago and the tips were curling before that so I'm not sure that would be the problem. But I will move it, maybe they are getting too much air. Some other information: I water my plants 2 times when the lights are on and once when off. Each watering for 15 minutes. Do you think I am not watering enough? The rockwol seems to retain a lot of water. I feed my plants 5ml per gallon of grow, and 10ml per gal micro, bloom, vegan tea, cal-mag.

You're overfeeding. Your leaves should be reaching toward the sky, not curling under (see below).

I feed my plants 5ml per gallon of grow, and 10ml per gal micro, bloom, vegan tea, cal-mag

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I take it you're not using a TDS meter? You're feeding at a minimum of 2.5 EC right, that's waaay too high.

That is probably what it is. All of the healthy plants are all 2 to 3 times the size as the ones that are curling. But two other ones smaller than the curlers are doing fine. My ppm meter broke last week and I am waiting for a replacement. I feed my plants generally what the bottle recommends but a little less. Maybe 90% of what the bottles recommend. Last time I checked a meter my nute solution was about 1400ppm. Also, I know E.C. stands for electrical conductivity but what would

You're feeding at a minimum of 2.5 EC right

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mean?

There are a few ppm scales out there so your 1400 may not be my 1400. EC is the same across the board.

http://www.hydroponics.net/learn/debate_over_ec_and_tds.asp

What's your humidity been at?