Death.

My day was decent, came home, ate dinner, then went to my room to find my poor goldfish stuck to the filter. That means one thing: it was dying. It didn't have the strength to swim away, and it had ended up stuck to the filter. The instant I saw, I felt bad. After some analyzing, I realized there was no way to save it. When I put it in a smaller container where it couldn't get caught in the plants, it just hung upside down in the water, gills still moving, but otherwise the fish was... still.  I helped the fishy pass on. It made me feel horrible, staring at it's lifeless body, it's eyes staring like they did, just like when it was alive.
I thought Doom might think me silly, with his joking insensitivity to my melodramatic ways, but he understood how I felt. How strange, I thought, remembering the cat that died, whose heart I saw pulled out of its chest, and how I hadn't felt sad, yet for this fish I felt something. It's different when you're the one who cared for it. I had saved this fish from death before, yet could not save it now. Aaah, death and learning. Foreshadowing, perhaps?