When I was first getting into fish keeping I bought a dragon fish. I did no research, had no idea what brackish water was, poor guy didn't last very long.
Bought a little, about 2" crab. One of my angelfish got it a couple days later.
got two convicts in a 75 gal with a oscar and they had fry every freakin month and hassle to deal with and they would tear up my oscars fins): so i let them go in a local lake