Hello I have recently set up a new tank which is now about 3 months old. When do I know its time to add
corals, and would zoanthids be the best choice to add?
You can get a few test corals. Some easy softies and mabye a hammer or something. 3 months in your definitely good to go. When I broke down my larger tank this tank was only about 2 weeks old and I added all these corals.
3 months should be good. I always had issues with corals earlier than that, despite on point parameters. I think the biome development helps a lot if you are starting from dry rock. If you have live rock, anytime after cycle seems to be good.