My first fish was a 2" White Catfish that my parents had caught when I was about 4 or 5. His name was Jaws and I adored him. Someone, and to this day I have no idea who did it, dropped a piece of buttered toast in the tank and it killed him. The butter actually solidified around his gills. I was so mad.
I had an awesome community tank that consisted of 2 goldfish (I don't know what kind. They got to maybe 3" and stopped growing), a minnow that survived a friend's Oscar for a year, a native cray and a 5" muscle. I had this tank for 4 years before something went wrong while the cray was molting and he died. Well, the muscle died shortly after and its shell popped open. The fish didn't survive it. That happened overnight, so we didn't know until the next morning. I was more upset about my cray and muscle than the fish.
After not having fish for several years after the mass death of my tank, my dad decides he wants to set up a water pond and add in some fish. He never asked me to look up which fish would do well in an outdoor pond in Alabama. He bought a bunch of comets, a pleco, and a pictus. The comets disappeared, one by one. I don't know if a raccoon ate them or if the pictus did. So a cousin gives me her "fancy" goldfish. One was 2 years old and the other was 5. Everything was great until the first time it rained. See, dad didn't drill holes at the top of this little premade pond. He just dropped it into a hole and called it good. He also didn't fill it back in around the sides after it was in the ground. So it rains and all the fish go missing. There's splashing going on in the gap between the pond and ground. The goldfish had gotten sucked in after they'd washed out. We didn't find the pleco and pictus until we'd drained the pond and pulled it out of the hole. Somehow the pond had lifted just enough for them to swim under it and then it crushed them. The goldfish didn't survive their gills being coated in red clay. I haven't had fish since.