What Hara said.
A little too much, too fast.
Your tank most likely hasn't had a chance to build up enough beneficial bacteria to cope with your bioload yet.
Did you use wet live rock or dry base rock?
If dry, then it will take a lot longer to become "live" enough to properly filter.
If wet, then it will begin filtering right away, but it will probably have at least some die-off that will create an initial ammonia spike.
How about your substrate?
Are you using live sand, dry sand, crushed coral, etc., and how deep is it?
Be prepared that there is a chance you may lose the other fish before your tank has properly cycled.
Some aquarists say that Bio Spira is worthless junk, but I've had good luck using it on my tanks in the past.
It's a liquid pouch of live bacteria that's kept refrigerated at your LFS
until you use it.
It's expensive, and there's no guarantee it will help, but if you can afford it, it won't hurt anything but your wallet to give it a try.
You also might want to consider taking the fish back to the store for safe keeping until your tank has fully cycled, and then add them back a little at a time.