best way to treat ich

[fishoholic]

QT a fish that is showing signs of ich in the first place is a good idea before adding to the display.

However if you already have ich in your display (like I did) I found that my cleaner wrasse got rid of all the signs of ich on my fish in three weeks, and a year later my cleaner is still keeping my fish ich free.

[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.