Healthy Plant w/ Brown Dry Bent Up Leaf Tips (PICS)

This is my first grow under CFL lights (276 watts) and the plants are doing extremely well except there is one 1 issue I can't figure out. Some of the older leaves have brown dried tips that are curling upwards. Not all the leaves have it just the smaller older ones and a FEW newer ones, other than that the plant is very healthy and doing well.

Strain: 707 Headband started from fully rooted clone in rockwool cube
Medium: Ebb & Flow System
Nutrient: Fox Farm According to week 2 schedule
PPM 750 currently but has been as high as 850 before res change
PH Kept around 6.0

Using RO water.

Like i said this is my first grow so this is a mystery to me. I don't think its nute burn because I'm following the fox farm schedule very conservatively.

Any ideas?

chemical burn.... too much nutes///

are you sure? Is it a MINOR burn? Should I lower ppm's?

Guess that makes sense because the newer leaves don't show signs of the problem, maybe they are more tolerant.

Yea, def looks like nute burn, whats the ppm at the mo?

fazman said:

Yea, def looks like nute burn, whats the ppm at the mo?

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I just check the PPM a 730. I would have thought that was low considering they were completely rooted when I bought them. I'd like to flower them within the week,