You said your salinity is 37...I think you found your problem right there for things dying. It makes sense your fish were alive for a few days and if you kept topping off with salt water like you said and it kept raising the salinity they would die. Also you need some power heads in your tank.
Salt doesn't evaporate, but water does. You need to be topping off with fresh water as others have said *RO/DI, not tap*
When the water evaporates your salinity will slowly rise as there is more salt to water, so you top it off with fresh to balance it back out.