4th week in flower unknown leaf problem please help

[tarandino]

hey people. i'm 4 weeks if flower. the strain is Big Bud and my setup is
600 hps sylvania grolux with reflector, 45 cm from the top of the plants
2 fans for ventilation.
Temps are from 78f(light on) 73f(light off)
2/3 soil,1/3 perlite at 10 littre pots
Water PH 6.5
Nutes Hesi at 1/2 strength
Half Tap water and Half Ro water

My problem start before 1 week when i see for first time some brown spots on the leafs and some purple stems. here are some photos

 

[Tokensmoke10]

Doesn't seem like a huge problem right now. I'd check your PH of water and soil and make sure everything is in line and just observe if it gets any better or worse. It just doesn't seem like it's showing enough yet to really pinpoint what the problem could be.

Here's a link to many common problems faced when growing:
Cannabis Plant and Pest Problem Solver - Pictorial

 

[DEAN420]

Looks like a cal/mg issue thats also locking out N

Up yr feed as you should be feeding more than 1/2 strength nutrients at this point and supplement with some cal or cal/mg.