Why did my fiddler crabs die?

A few days ago I purchased a 10 gallon aquarium along with other supplies to house two fiddler crabs (from petsmart). One was a female and the other was a male. Both of the them died the other day, and I’m not sure why. I think the mistake I made was that I over salted the water (I used the directions on the box for hermit crabs, 1 teaspoon salt per 250ml water) however, I recently got another crab and took some water out and refilled with freshwater, so now the water is brackish. could this be the cause for their death?
half the aquarium is land and half water and it was dechlorinated before i added aquarium salt. i boiled everything (shells etc) before putting into the aquarium and i do have a filter.
half the aquarium is land and half water and it was dechlorinated before i added aquarium salt. i boiled everything (shells etc) before putting into the aquarium and i do have a filter, also there is a freshwater dish for them as well