Wow, the stories of boxfish are sure true - they can wipe out a whole tank.
I thought I would take a chance with a small yellow-spotted boxfish because they are SO cute. I put him in a fish-only tank just like I was told to do. But, much to my chagrin, he got sick and died and wiped out all my other fish.
At first, I could tell that something was wrong with him because he was swimming strange, so I was hurrying to do a water change. (Hoping that would help.) But before I could get everything set up, he was gone, along with the rest of the fish. My tank appeared to get real cloudy and then all the other fish just flipped over and died - all within about 2 minutes of each other. It was amazing how fast it all happened.
Word to the wise (and I'm obviously not one of them):
1) if you have a boxfish (or cowfish) and it starts acting strange - take it out immediately. Don't think that it will be okay if you just wait. Throw it in a bucket, or whatever you have to do... just get it out of the tank as fast as you can. It's not worth the heartache, or the money, to risk all other livestock for one fish.
2) If you don't have a boxfish, keep it that way.