I have a 54 gallon established tank for 1 year and I recently bought red sea salt bucket and changed 10 gallons (as I do every 2 weeks) with RO water. I have 6 fish of different varietys and 2 pink shrimp and 1 red fireshrimp. I had birdnest coral, xenia and kenya tree. I changed the 10 gallons and over the past 2 weeks all my coral have died, my snails have fallon of the glass and died and my sea urchins have all stopped moving. My red fire shrimp had a problem moving around. The fish and pink shrimp don't seem to have been effected yet. I brought the water to be tested and they said everything is perfect. I tested the water as well and all results were good. I am at a loss what is happening here and what to do about it? Any suggestions?