Besides what I'm currently going through right now, one of the worst and saddest death I have dealt with was an African Cichlid I had for ever. I had lived in Texas and moved to Alaska for College and my grandmother took my African for me. When I had come back and got married, my ex insisted that my grandmother keep her. When I came back 6 years later, when my grandmother had passed, I found her tank sitting in front of a window, black with algae and only about 6 inches of water in the tank, no filter on. As I was cleaning the tank I saw a stripe of yellow in the green sludge, when I touched it it freaked. She was alive! I felt so bad for her. I carefully took her out and acclimated her to a 45 tall tank I had cycled at my new BF's house. The shock of all the light, clean water and space (she was in a 10 gal tank alone) She never fully adjusted from the years in that sludge. After a few months in fresh clean water, she died.
I don't blame my grandmother, she couldn't help the last few years she had, she had tried her best but the cancer was so bad, and none of her helpers knew that the tank held a live fish.
The other one involves the same fish and her bigger tank mate, my favorite fish in the world who I still to this day miss, my "Jellybean" Blood Parrot. I had gotten her from the horrid pet store I worked for and she was the last of the "Jellybeans" they had that survived. So I took her in and put her in my African tank, I figured that she would have been just fine seeing that my African was feisty and was 2 inches smaller than the BPC, all was good for a few months, with typical squabbling over territory but nothing to bad, added a huge log to the tank and that settled the feud..... at least I thought.... I came home from my first day of work at a new job hoping to see my babies do what they do (shifting the gravel around making little walls) and relax, when I saw a gruesome sight... my BPC shredded! Pieces of her all over the tank and her head laying in her flowerpot home still trying to breath, and my tiny African parading around a chunk of her in her mouth.
That little African was a tough critter and she was really my first fish. She survived a bunch of noobie mishaps and had stuck it out for almost 10 years.