browning on tips of leaves

not sure what strain this is but i am using a potting mix of 3 part organic potting mix watering twice a week and im using MG plant food until i get my foxfarm delivered im also using 4 6500k cfl twists 400 watt total there not to close so they shouldnt be burning plants.. plants are in a mylar grow tent with two small fans for circulation i have lights on 24 hrs ........ this is my first grow im thinking it might be from the MG .. any advice ? other then the spots on tips of leaves the leaves look very healthy i have experience in growing fungi so im pretty OCD bout everything so i hope this wont spread .. i also did a flush with distilled water and i will not be using no more MG/plant food until i receive my foxfarm plant food... thank you for your time and any input would be nice

thanks for the input budz will do