A few leaves are yellowing

I've got a hydro wick system setup, emily's garden. I germinated some seeds, and one of them exploded in growth over night just in tapwater and rockwool. he/she grew so fast that I didn't have the ph and nutrient solution ready for it by the time it had grown 3-4 leaves. It now has about 6 leaves. Two of the biggest leaves have definite yellowing and burnt tips. On wednesday I got the nutrient and ph to where it should be (I added 1 ML of nutrient solution, 1 ML of hydroguard, and the ph is about 6. The nutrient solution is botanicare pure blend pro, 3 1.5 4. I realize there should be more nitrogen then the rest, but thats what the shop guy gave me and I was confused. The yellowing and burnt tips occured before I added the nutrient and changed the PH. The PH of the tap water was 7.

The yellowing is occuring only to one plant, I have two other seeds sprouting, but they are in the first two baby leaves stage. the cotelyedon, I think they are called?