Many Questions?

Thanks for all of the input guys, things seem to be getting better.... I think. This is my first grow. I pulled out the rocks 1 by 1 and dropped the 1inch rockwool cubes down almost to the very bottom then replaced the rocks delicately 1 by 1. the cotyledons are still about an inch above the 3 inch netpots. Turns out my tub is 31 gallons and not 55. So I think the yellowing is nuteburn. I flushed and dropped the ph to like 5.9 it was 7.5, it has been 2 says since and the new growth is green and the roots are really developing,
the leaves that had the burn if that's what it was are slowly dieing. I don't know if the are supposed to bounce back or if my problem is continueing?
So lights are now ten inches above the tops and the temps are all right and solution is a little less then half strength and the ph is holding stable. If they pull through I am looking to get sixty clones into sog asap. What would be the best way to do this? is it possible? Here are some new pics. Thanks for all of the good advice. if anyone knows anything about this please let me know

Sincerely M.J Cox