Issue - Plant leafs have folded and gone brown

Ok I seem to have a problem with one of my plants and it is only one but I did have a second one that had the same issue, but had to kill it of as it was failing. Now this plant has come from a friends, infact all 3 of my original plants did. I have one that is doing really well and I'm soon to upload images to my grow log, the one that is failing I'm now at a situation of not knowing what to check next.

Now I've tried the following to see if it was any of them.

1. Switched from NUTE to just water for two weeks, but no change in the plant.
2. Checked the roots of the one I killed off and no damage to the roots, so no sign of root rot ( Second plant has gone identical ).
3. Checked PH level and its sat at 6.0, which is really good for the soil I'm using.
4. Checked level of PH in the water, which is at usually around 5.8 - 6.0 and the water is stod for 24 hours as well before being used.
5. Using the correct level of NUTES to water so certainly not over NUTING the water either.

Now my mate has had the same problem with his plants and we think it might be Thrips, as his new clones have the little corn coloured mite about 1mm long ( Not spider mite) and its leaving silvery tracks on the leafs. His havern't gone as bad as what mine have, but has the same signs as mine is having now but I've not found any bugs on my leafs, though probably not looked hard enough. What is strange about it all, is I have 4 very strong clones and one strong flowering plant uneffected by this, so at a lost end with what could be up.

SETUP:
600w HPS
SOIL
20ml Vitalink Nutes - 5ltrs of water (Its actually under by 12ml for Nutes)
Sprayed down about 30 minute before lights out, with just water.
Temp - flucs between 22c & 24c

Also my mate doesn't use nute and has the problem.

IMAGES:
Attachment 280020
Attachment 280020
Attachment 280021