Help a BUD out

Hey guys, i'm a nebbie to this so bear with me. This is my second grow, (first grow was with MG and turied out medicore. This grow is some ICE from female seeds. There currently in the 6th week since switching to 12/12. Here's my setup:

3 400watt MH to HPS conversion bulbs
using FoxFarm nutrients (currently Tiger Bloom and Big Bloom) using foxfarm suggested schedule
promix soil
pH solutions to 6.8
- haven't top, pruned or anything, just let them grow natural

temps have been 65 f at night, 78 during day
humidity 22-25% (very low, have tried to fix this to no avail)

Problem:
Just over the past week have noticed brown rusty spots on leave tips, tips curling up , believe this is a Mg def, have foiler feed with epsom salts and watered today with 1tb/gal of epson salts. This is currenlty only on what used to be my best looking buds on these two ladies. All other plants no signs yet.

Any suggestions? Don't want my precious ladies to die!

close-up

Cant really see from those pics, but try this:
Grow Marijuana FAQ, Cannabis cultivation - marijuana growing tips & photos

They look pretty darn good...
Not much you can do in week 6...

If it were ME... I would stop feeding and just use straight water....

cheers