In highschool, my family got a new pet fish: a little, black and orange cichlid named Oscar. The name was as unoriginal as possible, since it was in fact an oscar. Sort of the moral equivalent of naming a new dog Cocker Spaniel but hey, we had a lot of pets at the time and there's only so many names to go around. They can't all be winners.
We bought it to spruce up the fish tank a little, since most of our more interesting fish had died recently and we were down to a long finned goldfish and some guppies. The oscar immediately got to work, wiping out the guppy population in a few days and ripping the fins off the goldfish, leaving the torso to bob around aimlessly until it starved to death. This was already shaping out to be a great pet.
The oscar didn't get the tank to itself for long though. My family served as a sort of dumping ground for pets that no one else wanted, and it wasn't long before one of my Dad's co-workers had a baby turtle she wanted to get rid of and she decided that giving it to us was slightly less cruel then flushing it. Since we only had one tank, the turtle and the oscar were forced to live together. They were both only around an inch long and in no position to kill each other, the turtle wasn't strong enough yet and the oscar could only eat things that fit in its tiny, tiny mouth, so it seemed like everything would be just fine. We foolishly hoped that these two killing machines could live in harmony forever, killing wave after wave of goldfish for my amusement. Sadly, that was not to be.
The fish and the turtle grew at almost exactly the same rate, which unfortunately gave the turtle a huge advantage once they both got old enough to hate each other. Turtles can easily kill something the same size as themselves, oscars are still limited by how big their mouth is. Luckily, the turtle's head was just small enough to fit, something the oscar took advantage of every chance it got. I doubt this did any damage to the turtle, but it sure freaked it the hell out. With the turtle too scared to attack, the oscar got to live. Every once in a while the turtle would attack anyways (or just claw the oscar by accident, the way turtles swim seems designed to fuck up anything around them), the oscar would spend a few days on the brink of death and we'd have to waste money of fish first aid. This arrangement couldn't go on forever, we had to get our pets apart befor something terrible happened.
Once the fish and the turtle were both too big for their tank (almost a foot long each), we had a chance to separate them. The turtle was moved into a new tank, the oscar would stay where it was. This arrangement was great for the turtle, it was finally free of that terrifying bastard fish and had the whole tank to itself. The oscar didn't like it though, it stopped eating and even moving. Without the turtle around, all it did was drift around aimlessly in its now empty tank. Did you know fish could get depressed? I sure didn't. If we left the fish where it was it would starve to death, if we put it back with the turtle it was only a matter of time befor it got eaten.
We decided being eaten while happy is a better way to die then starving out from depression, so the pets were reunited and the oscar immediately returned to its happy, hungry self. The oscar held out a pretty long time with the turtle too, although the good times couldn't last forever and one night the turtle finally realized just how easy it would have been to rip the oscar to shreds all along. We found our beloved fish the next morning with most of its side missing.
There is probably a life lesson in all this somewhere.