bown leaves help plz

hi guys this is my second grow could do with some expert help..
im on 12/12 under a 250 hps in to 5 weeks on durban poisen..was on canna flores full strenth and 6 ml hammerhead, but i am just watering now coz i thought maybe it overnute..i have been doing a lot of reading on the net and keep coming up with cal defficiancy..have good air supply in and out water every 3-4 days when needed.and ph is 6.7 in soil..i hope somebody can help me..thanks in advance...