Hi and welcome to the forum
Can we get a picture of the tank, filter and fish?
How long do the fish live for after you get them?
How long has the tank been set up for?
Is the filter cycled?
Do you use natural sea water or use artificial marine salt to make salt water?
If you make your own, do you use dechlorinated tap water?
How long do you aerate the made up sea water for before using it?
How often and how do you clean the filter?
What is the ammonia, nitrite, nitrate, pH and salinity (in numbers)?
What brand of test kit do you use?
Nitrite is NO2
Nitrate is NO3
Make sure you use the correct bottles for the test