ALL my fish are gone. help!!!

hello everyone.

I have a 36x36 CMs marine tank...

It is about a week old. I love the tank.

I have 2 clownfish 2 small blue fish 1 blue spotted puffer 1 starfish 1 blue tang and 1 slug.

I have a 1.5kg live rock in the middle of the tank. Everything was going well until today morning... i woke up to find my 2 clownfish the 2 blue fish and the tang dead.. all thats left now is the starfish, slug and the puffer.

PLEASE HELP ME. WHY are they dying.. 100$ worth of fish dead.. i cant keep buying fish. they are so expensive. please help me :c!