How many times do i have to reuse the salt for my saltwater aquarium? ?

I have a 40 gallon aquarium and i am purchasing a 50 gallon bag of salt.


Um, you don't....

You add the salt once, top off evaporated water with fresh water (the evaporation is fresh water, and salinity increases as water evaporates, so replace evaporated water with freshwater, and only ever add the salt to the water change water), and only use more salt in water changes (mix the saltwater in a separate bucket).

A 50 gallon bag would fill the 40 gallon tank with saltwater and leave you 10 gallons worth of salt for a 10 gallon water change....

You will need a hydrometer to test specific gravity and confirm your salinity levels are correct.