Clam died... tank in trouble.. now what??

[Duffer2]

Okay, so I thought my clam was sick... I had moved it around in the last couple weeks, and it just didn't look right. Then all my SPS polyps didn't come out for a couple days. Today, I was working on my sump, and the smell from my water was repulsive. I took the clam out and the smell from it nearly knocked me over. The clam is now sitting outside my house as I attempt to freeze it, as even in plastic bags the smell was horrendous.

I did a 25% water change today and added fresh carbon. But things are looking rough!! I also checked my nitrites and oddly enough, they still didn't register on my test kit. Any thoughts or ideas.....