White Widow X Big Bud - First day of flower with burnt leaves HELLP!!!

[Skunkyflavour]

Hi guys have 9 x WWxBB Plants in first day of 12/12, they are 5 weeks from seed and going well.
have a slight problem with 2 of them atm.. first one is slightly burn / crispy brown on the top where the newest growth is coming from. all plants stand at just under 19 inches or there abouts.

any ideas guys?? can post pics if needed

check my journal for details of grow etc.

 

[KingJohnC]

what type of light are you using? how far is your light from the tops of these cannabis plants? how many millilitres per gallon and of what are you feeding your plants? what are you adjusting the pH of the water you are feeding? can you please post some photographs?