We bought our first salt water aquarium 4 months ago, a 56 gallon tank. For two months we had 6 damsels in the tank and finally felt good about the levels and that the tank was established well. At this time, we exchanged the damsels for two clown fish, a coral beauty angelfish, a blue tang, some crabs, some snails, and a hard corral and a soft coral. After a month with everything going well, we added in a blue tang and diamond gobi sand sifter. The blue tang died the day after adding it to the tank and we bought another after a week of waiting. We added in the new blue tang to the tank along with a star fish and a cleaner shrimp (we have had some brown algea problems throughout the process). Within 24 hours of adding these, all of our fish died (the snails, crabs, shrimp, starfish, and coral are fine). We have done every home test we can think of and everything appears fine. We took in a sample of water to our salt water aquarium store and they can't find anything wrong. They have checked our setup numerous times and can't find anything wrong. They then told us to change 50% of our water with some from their store and wait a few days. We did this and were then told to try 5 damsels again to see how that went. Within two hours of adding the damsels, they all had died!
As of now, there are no fish in the tank and we have no idea what to do or what is wrong (and neither does our aquarium store). Please help! We're open to any ideas, thanks so much! Sorry if I missed some details or had some wrong terminology (I'm new to this).