I agree with raising the salinity, for the ick protozoa to implode, you need a salinity of at least 3ppt (parts per thousand).
And you may need to treat the tank for 3 weeks to a month to get rid of the all parasites. Each spot on the fish morphs into up to 100 new juvie ick, and the higher salinity (osmotic pressure) makes them lyse (implode). Some are dormant in the substrate which is why vacuuming is important. Just make sure the replacement water also has salt added. You "don't need aquarium salt, any salt will do. As long as its NaCl, salt, is salt.
I buy 50lb bags of water softener salt from the local home improvement store, and it works fine, and is much less expensive. And it won't change the hardness your tanks water to any significant degree.