I usually flush 1 week prior to harvesting, if you start before that the plant will use the nutrients in its leaves and they will start to turn brown usually the bigger leaves but from the leaves none the less. Thats my 2 cents...Good Luck!
If you want to help speed up the process, get a warming mat put it under your tub and leave the sprayers on constant. Thats what I do and I see roots in usually 7-10 days. Good Job!
I wouldn't put directly under it to start off, ease them into that bright of light, then I would start off a good 3ft then see how they do, if they start to stretch then lower the light.
Looks to me like they need too start getting about 1/4 strength nutes, since your soiless mix has nothing in it the seedlings are looking for some nutrients........thats why if you look at them, the bottom leaves are turning yellow and the tops are still green, if it was nute burn all the leaves...
Kinda looks like the starting of a magnesium defiency to me, a ph problem would show more yellowing of the leaves. But you need to check the run off of the soil to find out if its a true ph problem. Just my opinion.......
Big bud is a pretty good strain, not as strong as say white widow but pretty good and fairly easy to grow with a pretty big yield, my last BB I grew I got over a pound off 1 plant. Just keep your nutes & ph in check and you should be fine.
Good Luck!!!!!!
Need more info.........could be ph probably too high, not low......could be nute burn, could be a mag deficiency. What are your numbers, you really need a nute & ph meter.