Brown spot in leave and margin

Hi, I need help identify my plant problem. I found burnt edge 2 weeks a go and put her on 200 ppm but 3 days ago founf this brown spot whcih look like can mag defi for me . Now i dont know what to do to feed or to hold. I use sensi bloom by AN and it has been cause trouble ever sincr as when i put up to recommendation 4 ml per lite r all my plant are burnt. I use DWC and this is white Russian fast version. Thank you fro all your help
 

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