You shouldn't be adding anything you don't have a test kit for. Otherwise it's easy to get things out of balance.
My work nano hasn't had anything added to it except food since I started it nearly a year ago. The weekly 1g water change has been enough to keep things healthy and thriving. So you may not need to add anything.
The most common additives are calcium, buffer, magnesium, strontium, iodine.
Be careful with iodine, especially in a nano. A little goes a long way.
B