Fish died overnight

[DeniseAndy]

If you have nitrite, you had ammonia, and unfortunately that will kill the fish. Nitrite is bad for fish. First off, I would make sure your tank is fully cycled prior to adding anything else. There are many threads here to help you figure that out in a method that works for you.
Next, I would reconsider your inhabitants. A 13.5g will not support a diamond goby or sand star for long. Look at little gobies, maybe a small blenny. But not until you are sure your tank cycled.
Good luck. I hope you can get the tank stable and mature as they are a fun size tank, but need to be very careful to keep stable.

 

[Espielozano]

Did you find the starfish alive? Because it will die in such a small tank at some point...not if but when. A sand sifter dying in a 14g could wipe the fish out...that is very reasonable.

If you find it alive...you should return it before your add more fish.

Nitrite is not very dangerous in salt and 0.25 is nothing.

The starfish is alive. He was scooting around while everyone was dead ): but right now he’s kinda arching himself if that makes sense. Hes on his tip toes right now