Exactly. The challenge with a new tank is:You dont have to wait 6 months to add coral. You do need to acclimate corals to your tank parameters, alk, calc, nitrate, temp, sg, and lighting, flow. Its not great to have corals and go through the algae phase sure. If you are on top of it with correct cuc or manually its ok. Its not because this tank was set up a month ago. I'd like to see pics of the corals under white light before and after addition and actual parameters of the above before I would be saying its because its a new tank. I mean its been 1 day if the hammer was ok truly ok when purchased its not in that shape from it being a new tank that is actually cycled.
1) parameters tend to be less stable. So sometimes big swings in Alk, N, P, etc. corals don’t like this - much easier when they are steady (sometimes regardless of the number unless it’s dangerously high or low).
2) algae - your going to get the ugly phase which will cause swing in parameters (#1), and also potentially crowd or smother your new frags. Need to be on top of it
3) lighting - as others said, acclimate and start low. You don’t know the old setting , so start low and you can also just place them lower in the tank to start.
4) you start with a smaller micro-biome. So the sponges, pods, filter feeders and other stuff that help a healthy aquarium are not there. If your starting fresh (no live rock), they only way to really introduce that is to go out and purchase it explicitly or start adding some coral frags slowly.
I don’t think you need to wait 6 months, but need to take into account 1-4, start slow, pick some easy coral (like you did), stay on top of maintenance.