leaves have brown spots

i am on my 3rd week. im using adv. sensi bloom, big bud and carboload. the leaves started off by getting these white dots so i searched for mites and nothing. i have been checking for bugs everyday but nothing is there. so a couple of days ago the spots started to become copper colored that where raised and then today the tips started turning brown like a burn. my nutes are at 1200 and my p.h. is at 6. . is this a magnessium problem
 
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