Green Chromis

Is there something in their diet that I'm missing or are they rental fish? I started off with 3 then they slowly died off over a year.

 

No they can be fairly hardy but easily picked on by tank mates. They are prone to uronema marinum, which kills them. Try adding selcon to their food.

 

It's like saltwater ich where the fish has to fight it off?

 

Nope a completely different pathogen.

 

They often are also brought to the surface way too quickly which damages their internals. Some will die quickly from it, others will make it to a store and/or your aquarium but die within a month. Even if they live longer, I wouldn't be surprised if they still could be weakened somehow from it and die more easily than ones caught properly.