fish dieing

[civickid]

about a week ago it all started this tank has been up and running for a month and a half now. the first thing to die was all of my snails started dieing off then my yellow tang gor red on his lips and gills then last night i had a banner cardinal die. what should i do
nitrate 0
nitrite 0
ammonia 0

 

[beth]

How long has the tank been set up?
Is there any chance some kind of poison from the outside the tank got in?
Do you have any corals in the tank?
Notice any disease? What was the last fish in the tank?