"Rusty spots" please HELP!!

My girls are AK-48, in Soil, Fox Farms OF, 3 gal. pots, under 400w HPS, water,feed,water feed,ect, watering every 4 days, Flowering now for 31 days, total days growing 61, there doing fine growth wise put ive been seeing "rust spots" on the older fan leaves closest to the buds top of plants ,(35 in. tall) more on two of the 4. SORRY NO PIC!! camera broke. Im thinking cal. or mag. or both but not sure how to fix. next watering will be just plain water Ph'ed at 6.8, my run off is about 6.1, i know thats low but in trying to bring that up slow so i dont shock the plants, its coming up, 6.5 is where i'm shoting for. ANYWAY please help with "rust spots!!

Get some pulverized dolomite lime and put some on the top of soil around the base of plant or mix it in with water. Should keep your PH at 6.5 and take care of the cal mag problems

yeah that sounds like iron lock out your PH is probley high like the guy above me said lime or egg shells for long term ...A tea spoon or two of lemon juice or ph down per gallon might be a quick remdey for ya