All the fish have gone through quarantine after I had ich twice in the tank. The last fish, which was added about a month ago, did not. I got a bit frustrated as three fish died in quarantine, so I took a chance. Now I see it scratching its head/side against the sand (sideways), not sure why, but it opens its mouth as wide as it can when other fish come near. Since this is a diamond goby, it's impossible to tell if it has ich—its fins look fine, and I don't see anything on the other fish (at least not yet). Any suggestions? Should I remove all the fish or just the diamond goby? Copper or another treatment?