Nobody seem to have given you good idea on what to do.
I was a pet care specialists for 2 years. People would actually call me when I was off work for advice.
Heres what you do.
First step should be raising the temperature. I would raise it up to 82 degrees.
During the same time put in some salt. Dont use table salt. You want salt that is used for water softener systems or aquarium salt from a pet store. Both are the same thing. Usually the directions will tell you how much to put in, it will usually make it anywhere from .2% to .3% salinity.
Why do you raise the temperature and add salt?
Ich is a parasite that is developed from many factors. In cold water, Ich will be more common than in warm water. Ich has a hard time surviving in warm water. Thus raising the temperature will help eliminate the Ich.
Also, Many parasites will have a hard time living in water that is saline. This why you add salt, It will help in the aid of destroying the parasite.
I personally leave salt present in my water all the time because it aids in preventing diseases or parasites.
Just make sure you have lots of oxygen supplied, as time like this the fish is stressed out, so just having a good supply of oxygen would help.
Make sure there is a current in your fish tank. Raising the temperature and adding salt weakened the Ich. Now you will need something to aid it getting the Ich off faster. Use a pump to make current. The Ich will then get blown off of the fish by a strong current. Not too much not too little. Just use common sense here and figure out how much current is needed, as it is different depending on the location of the pump and the way it is pointing.
Now that you have that done.
Do water changes every other day. Not from the top of the surface, but from the bottom with a gravel vacuum. Do a 25% water change every other day. This is very important that you do this. Water changing is your cure!
Then do some math on how much more salt needs to be added to the new water so that the salt level stays the same.
Keep repeating the water changes and I am 100% sure that the Ich will go away in a week or two. Ich is soo easy to get rid of.
Once you see that the ich is gone, replace "all" the filter cartridges. It is important that you do this because there will be bad parasites in the filter. Get new ones or later on the Ich might re appear.
To all of the people that I have given this advice to, no one has ever come back to me and said it did not work.
Goodluck and let me know how it turns out.