Need help with Quarantine tank

So a week ago my royal gramma that I had been nursing in my quarantine tank finally died.  I had been nursing it for 2 months.  Within 24 hours, the bangaii cardinal and a new royal gramma also died (also in the quarantine tank - new fish I purchased two weeks earlier).  I'm guessing it might have been an ammonia spike - though my test only showed between .1 & .2, and nitrate was at 10-15.  Or maybe the old royal gramma actually had a disease.  Never knew. 
Anyway, I want to get the quarantine tank set-up again - and that's what I'm looking for advice on.  I think that it would be best to totally empty the water and clean it good (with vinegar or bleach??)  since I'm not sure what caused the death of the fish.  I have one small piece of live rock in the tank.  It was added only about a month ago, to help with the bio-load.  There is a mushroom on the rock and it still looks good.  Do I need to take any precautions with the rock?  I don't really want to keep it in the quarantine tank.  Can I add it back directly to my display tank??  Does it need to be dipped or something??  Should the sponge filter be cleaned well also - destroying any bacteria that might have accumulated?  - then put it back in my display tank for a week before I start the quarantine tank up again?  Or since the sponge is so cheap, just discard and use a new one?????