Its always best to use RO water.
Get a 10 gallon tank w/heater, biowheel, and powerhead.
I would use hyposalinity as the method of curing the ich.
Ich cant survive without a fish to host it. This means your display tank will have to have no fish in for 6-8. Now for that same 6-8 weeks you fish need to be in a hopital tank undergoing hyposalinity. Your tank salinity needs to be brought down to 1.009 over a 36 hour period. When it is at 1.009 the 6-8 week period starts.
You will want to do this with a refractometer. It is imperitive that the measurement is accurate. You can not use a swing arm device.
Start by doing water changes and replacing the water with fresh water NOT salt water. This will drop the salinity.
Your starting point should be between 1.025 and 1.027. Replace about one fifth of the volume with RO, RO/DI or aged freshwater that has been well aerated. Repeat this 12, 24 and 36 hours later, monitoring the specific gravity along the way. After the fourth water change the specific gravity should be 1.010 or pretty close. Wait a few hours to make the final adjustment to get down to 1.009. Note that you can estimate the resulting specific gravity. If you are changing one fifth of the water and the current specific gravity is 1.025 the result will be:
((1.025*4)+1.000)/5 = 1.020 approx.
Then, after 12 hours:
((1.020*4)+1.000)/5 = 1.016 approx.
After 24 hours:
((1.016*4)+1.000)/5 = 1.013 approx.
After the 4th change:
((1.013*4)+1.000)/5 = 1.010 approx.