depends on the type of coral. Usually it will be at least 2-3 months before your parameters stabilize with a new system. So add an easy, hardier coral after your parameters have been stable for at least a month.
I was successful with a few Zoas after a month or 2. You could even do a mushroom. When it's a month or 2 throw in a mushroom and a Zoa and see how they do and go from there