Short Story 90%+ of your problems growing weed indoors is due to having an incorrect PH of your water or feed, read on for a story and explanation.
I'm on my 4th grow and after having only 1 major disaster last grow with white flies(arrgh) I've got some advice for you.
When you have a problem with your plants , every experienced person here is going to ask you what is or to check your PH of your nutrients or water
LISTEN TO THEM!
I have some clones of my last grow(separated with NO white fly) and after I had cleansed the grow room of the white fly I put them with a 1/4 strength nutes, the ph was right and after a run or two the EC was perfect( more on that later).
I knew the ph of my water that I top it up with is about 6ph, normally perfect and with nutes brings it down to about 5.7ph.
They powered for a week, growing like they should
Then it started to rain and rain and rain.
At this stage I made the mistake of thinking that the ph of the water would remain at 6.
So I kept giving them some water every second day and feeding once a week.
After about 7 days, they started looking pale but my EC was right. I thought what the fark, those little white fly farkers are back again, I checked the specific white fly symptoms but to my surprise I just couldn't find any at all.
I was stuffed, I knew I had a problem but couldn't work out where, I knew my EC was right and I knew my Ph was, hang on a tick I haven't checked it in the last 2 weeks ,just assumed.
So I ph tested the tap water and to my horror it was 7.5 ph .WTF?????
All the rain had significantly raised it and I'd solely been giving this to my plants and with the little nutes I was putting in it, didn't bring it down enough.
I adjusted the PH and boom off they went.
Short story, if you don't have the correct PH for your medium(outdoors, coco, hydro etc) your plant cannot get any nutrients at all.
A short note on "EC readers".
Serrated edge came to my help on this issue, in basic terms via electrical pulses these readers tell you how much nutrients you have in your feed.
Too much is worse than not enough, at different stages the plants need more( ie in growth and stretch periods) and less(in the clone stage, you make a higher ec reading slowly as they grow and at the end of flowering).
I'm still learning with the EC