New growth twisted, tips yellowing...

Hiya
Can anyone identify whats happening here? New growth is twisted, not growing nice and straight. Leaf tips also browning. Im using a diy led (4 x cree 2530 cobs) which doesnt put out so much heat, light is 5 inches from plant so i dont think its heat/light stress. Im in DWC, pH 6 and fairly stable, ppm was 700, dropped to 600 overnight so im fairly sure its not nute burn either.
Any ideas?
Cheers

Light is way to close
move to 20 inches and see how it responds

Make sure the pH number is true. Is your meter calibrated?

Yup meter is calibrated.
Ok ill move the light and see what happens.
Cheers!

Moved the light further away, new leaves seem less twisted but tips still very noticeably yellow. Ppm 850 fell to 770 overnight, pH5.7.
Thinking of trying a semi flush at 400ppm...
What do you think, any ideas?