I always considered them deadly from my freshwater days I would never add fish with nitrite present. I could wait the couple weeks to be sure the cycle was completed
Nitrite in freshwater IS deadly.
Welcome to saltwater where it DOESN'T matter. It's not toxic unless it's off the chart.
Don't test nitrite anymore. Throw that kit away.
You are cycled.
There is a reason you're fish died, there always is.
Can you list all your parameters?
Do you have a quarantine system in place for all new fish?
My best guess is disease brought in from your store. You will need to go fallow in the mean time.