Simply leaving the tank alone for two months won't do anything, you have to keep your eyes nitrates,nitrites, ammonia, etc. I assume you were watch all the levels too and forgot to mention you did? When you added the fish was ammonia and nitrites at 0? And the nitrates at 10 ppm
if not lower? Adding more live rock will add some ammonia to the so waiting like two weeks is a good idea. But before those fish touch the water make sure those levels are right! You may have acclimated the fish in a FW
way and not really a proper way, but they should of died a lot sooner if you did it all wrong. I believe it's from cycling. You will often hear people say fish in an un cycled tank won't last long. But they will live for a good 2-3 weeks. And your fish died what 3 weeks after? You did cure all of your live rock too right?