The problem is that your nitrate is not truly zero. I would bet money that there is algae and/or other organisms living in your tank (probably on your rock work) right now that are using the nitrates up and preventing you from being able to detect it in your testing. I remember when my tank was 2-3 months old I had this layer of thin, fine brown algae on the bottom of my tank (it's barebottom), that stayed there for weeks, if not months. During that time my nitrates were always zero. As soon as my snails and fish finally rid the tank of that algae, that's when I started seeing my nitrates test above 0. It took months for that to happen.
It's really best to let nature take it's course. Your tank is too young for corals anyway. They won't thrive in these conditions, even if you dose nitrates. It's best to take a pause, don't add anything else, and let the system stabilize. Once you start seeing strong coralline algae growth, that's when you know the tank has reached some stability/maturity.
I also recommend running a strong UV sterilizer during these initial months to help combat major algae/dino/bacterial blooms. Continue doing beneficial bacteria as well. This really help with the ugly phases.