You can use solar salt, sold for water softeners. You want the crystals in the blue bag, 99+% pure salt without any additives. Avoid rust/iron fighter/remover. Places like Lowes and Home Depot have it.
I suggest adding one bag at a time, giving it a day to dissolve, and seeing how you like it. Then try another and so on and see how things go. Another approach is to get the AquaChek salt test strips, measure your current level, and add salt to about 2000 ppm.