Ich

I have 3 hermit crabs and a starfish. Can I just take them out and treat the fish with copper for ich?

 

No, you should never use copper in the display. The gravel/substrate and rock will absorb it and you'll have a tough time keeping a therapeutic level. After treatment you would not be able to use the rock and substrate again as copper is extremely toxic to inverts.

 

ok thanks

 

You're welcome.

What has ich and how severly and for how long?

 

I had a long nosed butterfly fish that recently died from ich... I wanted to get rid of the ich so the other fish don't die. 1-2 weeks

 

I'm trying a water change right now.

 

http://www.3reef.com/forums/fish-diseases/life-cycle-marine-ich-96343.html#.Uwqp-oUcHIU

Since that thread has been posted two other options are available one being the transfer method.

Marine Ich - Myths and Facts

The above thread is important to choosing the right treatment.

This one is very good as well.
Marine Ich/Cryptocaryon irritans - A Discussion of this Parasite and the Treatment Options Available, Part I by Steven Pro - Reefkeeping.com

You can not treat or eradicate the parasite unless you understand the life cycle and proven treatments.