My worst fish was one I didn't actually buy. I bought a 44 gallon corner tank through an ebay auction and they asked if I wanted their 6 fish. Without thinking
I said sure and aquired 5 nice fish and a devil. The devil was a 6" pictus catfish that would eat anything it could fit in its mouth, including 5 of my 10 long finned danio that I moved from a 5 gallon tank to the 44.
It took me several days to figure out my fish were disappearing as its hard to count the fast swimming danio through plants. Finally I did and I had a horrid time catching the catfish and then it got its barbs stuck in the net (and nearly my hand). Thankfully a local fish store took it, and boy was that thing FAT
from eating my danio. Glad to see the last of that fish, and yes, I am now much more carefull about checking out any fish that I acquire.