Third Times A Charm (1st Grow)

Growth is very strong but there are issue starting up...firstly let me start by saying I am growing in FFOF with about a solo cup sized amount of Happy Frog where I started the seedlings. The Wedding Cake auto has begun to show signs of flowering but also signs of what I believe is nutrient burn (but maybe P deficient??) and early signs of nitrogen toxicity and I have not fed them any nutrients except CalMag with RO water but could that cause an issue? The Blueberry also shows signs of what I believe is nutrient burn but not as bad. Both plants oldest leaves are beginning to yellow, not sure if that's normal or a sign of something. But they are still progressing quickly apart from these issues.