Third week in veg spot on leaves and red stems

Growing in dwc single buckets using four gallon of reverse osmosis water and a ounce of a&b sensi grow ph perfect ppm is at 720 the ph star at 5.8 in the first day and up to 6.2 or 6.4 in 24 hours temperature of the water is at 68 to 70 the funny part is I have ten Larry og only four have this problem this is my third time growing having the same problem for real it's time that someone open a school to teach us how to grow step by step. really need help

please take pics in white light.

Get some Cal-mag what is your PH.... ? its a calcium deficiency

People say that advance nutrients a ready have calcium I call advance nutrients support they told me the same thing. I change the water every week my ph start at 5.8 in two days is at 6.3

it looks more like magnesium def to me. I've seen it before and it looks a lot like that. according to the advanced website 4 ml are to be used every liter, but there are 4 liters per gallon. so this means you'd have to use 4 times as much as you would for a liter, which means 16 ml per gallon. there are 5 ml per teaspoon so for a gallon of water you could use 3 teaspoons to make 15 ml, as 5 ml x 3 = 15 ml. this is one ml short of 16 but it's kind of unlikely missing that one would hurt. I don't know if your 720 ppm equals 15 ml or not.
http://www.advancednutrients.com/products/ph-perfect-sensi-grow-parts-a-b/

toward the bottom of the page there is a guarantee that if you don't like the product, you can return it to the seller with a receipt for reimbursement.
the website says you use 4 ml of grow and 4 ml of bloom. the danger is that in order to make sure you use enough, you can easily over do it. try to make sure that either A or B has magnesium in it.

Thanks one more question do I need to add cal-mag every chance of water or when the plant need it

I'd just give it once with next watering and see what happens. you don't necessarily need to give calmag with every watering. not sure how people latched onto this "give calmag" to solve every problem. it really looks like mag deficiency, but good ferts should have cal and mag in them. so read the contents of you fert.