dying fish help!!!!!

[Aarcox]

I have a 180 gallon with a few remaining fish, I moved my fish to my back up tank for sick fish. But I had a French angle grey angle, and a young banded cat shark all dead over night! I took my water in and everything tested out fine. And all the dead fish seemed fine the night before, the remaining fish I have are a clown grouper, two black spotted groupers and a small snowflake eel. I have about 30lbs of live rock and live sand, the tank was set up about two months before I had any fish in there and all the fish did fine for about a month. I did about a 30gallon water change after the died two days ago but scared to put the fish back in. Any suggestions or comments?

 

[Aarcox]

And also I'm running a fx5 cannester and a little filter rated for 50 gallons just for some extra water movement