As a child: Bought a bunch of guppies, serpae tetras, buenos aires tetras, siamese algae eater. Stuck them in a 10 gallon with only a filter and plain tap water. Guppies died within a month, everything else lived for at least a year. I then threw some crayfish from the local creek, took out everything, a bunch escaped the tank and crawled around the house. My grandmother picked one up thinking it was a big bug and it pinched the crap out of her, that's still good for a good laugh 8+ years later.
As an adult Hobbiest: Jack Dempsey Cichlid.
As an adult Hobbiest: Jack Dempsey Cichlid.