my worst purchase was a green spotted puffer. I had it in the aquarium for about a week and everything seemed fine. Then i started noticing that some of the fish were missing eyes so one day i was watching the fish for like an hour and i saw the damn puffer bite the eye off one of my plecos. soon all 10 of my fish were dead and i ended up with one fish. It killed everything including an australian arowana. I had a separate tank with a huge pacu and i fed the puffer to it and the pacu ate it and almost immediately flipped over and floated to the bottom and the puffer swam out of its mouth. This was stupid but at the time i didn't know the puffer could do that.
Unbeknownst to him, he made a hasty retreat back to the lfs!!