Aquarium Supplies World

I have a 150 gallon saltwater aquarium and want to convert it to freshwater. Can it be done and if so how do i clean it?

I have a 10 gallon tank with one betta, four mollies & two golden mystery snails. I have read that both bettas & mollies like a small amount of aquarium salt in their water. Will this also agree with the snails?
If so, how much salt do I add?

I just bought a new fish tank that I’m filling with freshwater fish tomorrow, but I need to know if its absolutely necessary to have a canister of aquarium salt in there. Petco does it. I’ve never seen it done before so what, if any, are the benefits?