Had the same prob last year but it turned out my grow space was falling below 10degre at night.
That is weird to just have one leaf white. isnt that iron deficiency tho? If you can test your soil ph that would check one off the list. I usually nvr worry about it bc i have ph perfect advanced nutrients. i dont think it could be light burn if its just one leaf.....what lights are you using? I think i had my led light too close myself.....and deficiency.
My temp gets up to about 80 sometimes and drops to about 71 at night. so your saying even though its not "cold", the fluctuation can mess with the plant?
Id like to see a pic of this. Purple could be lots of things, phosphorus definciency, or it could be the strain. post a pic and lets see
I run my plants at around 80. 70-80 degrees is typical. How do you think Mexican and African Sativa and Middle-Eastern and Himalayan weed grows? 23-27? Degrees celsius is optimum range.
my ph is now 3.5 and all my shit is dying im 6 weeks into flowering and my ph keeps going down :/ i rly need help guys i dont knkow what to 
do pls pls pls help only 23 days left now 
help help pls
Holy fuck, dude, use some pH up and possibly some cal-mag or epsom salt diluted in water. 3.5? Jesus..help help pls
Hey, you're right. He's like, 5.5. It only needs a little bit of pH up or something. Should be alright.
All 8 of my plants turned purple. There's 3 different strains...1 is unknown and I have cookie monster and original sour diesel. After not growing almost at all I cut some of the leaves off. I am having trouble up loading a photo but I'll figure it out soon. Thanks for the reply
Oh hey.....if you go to the top of this page you'll see one of the plants a couple weeks ago. As u can see it has a couple more layers now but overall way too small than it should be.......but that pic is the start of the purple. The one I just posted, I just took. These plants are like 34 days old. I've nvr had it this bad. ):
yeah that was the day before now its 4.0 :/ dont rly know what i can do i did nothing wrong some people told that foxFarm nutes makes ph go down i tested ph 1 weeks after flowering it was 6.6 at start/Vegg ph was 7 watter i use KRISTAL Water drinkable water feed nutes twice a week only and 2/3 water any help what im doing wrong or any ides why ph go down so dramatically i only have 23 days to finish im losing them each day
using 600w hps in a cooltube 1.2 tent good air flow in and out 3al pots seeds are sensiseeds big bud