My leaves are turning bright yellow??? (PICS)

Ive noticed this happen over the last week or two. I havent really changed any routine to what im doing ie. water with boiled then cooled water every three days, 12/12 lighting about 6" above plants (250w Sodium).
The plants are 7 weeks old now 4 weeks veg.
Temp is around 81-85c during light and 65-68c in dark.
PH 6.5-6.8
Humidity around 52%.
AK48's from seed

I was using Bio-Grow fert, but my plants didnt seem to like it so stopped using it around 2 weeks ago. About 4 days ago i tried using it again cause my plants looked ok (so why did i use it i know!) but you always think it makes them bigger and better i suppose. They started dropping again so i decided im not gunna use it again so i flushed them out with pure water. This isnt the prob is it?

Any advise would be good cause im pretty proud of me first grow. Dont want them dying on me now!!

Same thing happened to me. It appears to be a nutrient deficiency, most likely nitrogen. Fairly common issue at this stage. I've learned that you want to keep those fan leaves around as long as possible. What is your nute schedule?

Ive stopped given them Nutes altogether. They just get pure water all the time now, well from about 4 days ago.
When i was given them nutes i feed them once per week. 2.5ml to every ltr of water.