A giant gourami (Osephronemus gorami
)- thought it was a great deal for $10. About 2 days later he ate $50 worth of my other fish. Then I had to get a new filter ($150) to set him up in another tank.
I did end up keeping him for several years (he grew to almost 2 feet), but had to give him away when I moved. He went to a guy whose family ran a Chinese restaurant and apparently had a 200+ gallon show tank. Afterwards I read somewhere that these are a prized food fish in Southeast Asia... maybe he became the Sunday Buffet Special. I never had the heart to go check