Well, I must say, I 've stopped posting here, when you decided to continue to dump copper into a new tank. A tank that has just been set up in June cannot possible take any type of copper treatment, let alone copper for 3 weeks, for godness sakes.
No, I'm not here to say I told you so, because what's happened to you and your fish is too sad.
Obviously your LFS resource is a hack, and you need to not only stop going there for advise, but also stop patronizing that store. That store is not only selling you a bunch of junk he's also spouting a lot of bull just to get you to make purchases.
How many fish do you have left? Do they still have ick? I'm sorry, this thread has gotten pretty long, and I've lost track of the events.
The first thing I would do is setup a QT for your remaining fish and horseshoe crab, whether they have ick or not. You can do this pretty cheaply as all you need is tank and PH's. Use the skimmer on your current tank to filter your QT. QT will not need any lighting in your case.
You need to break down your main tank entirely after you have moved every fish over to QT. No substrate in QT. I setup a QT in May, 20gal and put a few pieces of cycled dead coral in it. It never had ammonia or nitrite, so it can be done. Make no mistake. You need to start over with your tank.