Can you use water softener salt in a marine tank?

In general that is not a good idea. It will contain too much sodium or potassium.

When you add the salt water mix to the water, the manufacturer expects the water to be free or nearly free of these.

The extra minerals will change the salt water into one that has too much salt or too much potassium.

Having said that, I think there are some people who do use it and do ok.

It probably depends on how they make their salt water and what they put in the tank.