I would clean it up, that is, cut off the worst of the yellow leaves, and trim the bottom leaves (helps with bug control also), and leave it at 400 for a couple of weeks, and then slowly raise it, keeping it around 600 in Veg.

Once in flowering, I try to keep the nute level at 600 to 800 for the first week or so, and then slowly raise it to around 1200 till its time to flush
SpaceNeedle Reviewed by SpaceNeedle on . 3 weeks from seed and tips yellowing?? 5 ladies in Bubbleponics setup under 600w MH 30 inches away. Ph average 6.0 to 6.8. , Temp 73 to 79 constant.They are 3 weeks from seed. GH nutes, roots white. Any suggestions why my tips are turning yellow? Please help. Rating: 5