Well I can tell you that Ammonia was not in your replacement water, so unless your poured it in there, I truly believe that was a random thing and that your problem has been festering for a couple days. Something is raising your Ammonia...how long did that dead fish sit in the tank? A water change(s) will cure just about anything other than disease so I would highly recommend you do that...most on here will agree with me on that. They can NEVER hurt.
Clearly their is something in that water...prob high levels of Ammonia...that is killing your precious fish. Your two most delicate fish are dying or dead...damsels and clowns for the most part can survive anything so do that big water change. Run a 2x and 3x test for ammonia a check it to make sure its correct. Also check your Nitrite again?
Is the LR new? What all has died in the tank?
Clearly their is something in that water...prob high levels of Ammonia...that is killing your precious fish. Your two most delicate fish are dying or dead...damsels and clowns for the most part can survive anything so do that big water change. Run a 2x and 3x test for ammonia a check it to make sure its correct. Also check your Nitrite again?
Is the LR new? What all has died in the tank?