best way to treat ich

[0sprey]

I'm a bit concerned about the length of time this tank may have been running...
You said the clowns were added 3 weeks ago, and the tang 1.5 weeks ago?
How long ago did the tank finish cycling?
What are the water parameters?
Did you quarantine?
Usually a severe ich outbreak is as much related to stress as anything else. Healthy fish in healthy living conditions don't normally have any trouble fighting it off on their own.