I suppose this counts as neglect. I was young, had just gotten into the hobby probably a few months or a year before this incident. Did little to no research because I had no idea wonderful places like this existed.
Anyhow, I lived in upstate NY at the time and went to the city to visit my grandparents for what I think must have been an extended weekend. I fed the fish extra prior to leaving because I was so afraid they would starve and then tossed in one or two of those vacation feeders. A few days later I come back to an unimaginable sight. Algae covering nearly all the walls. Grime and gunk I can't imagine littering the substrate and some slime at the top of the water. Fortunately, I don't think I lost anything. If I did, it wasn't much. And it's nothing that a few water changes and some scrubbing couldn't clear up. But dear god what a sight.
This next one unfortunately did result in a death and comes to us courtesy of my sister and father. Somehow we both had what I think were called "firebelly newts." I'm sure they have another name now and probably did back then as well. We were both younger than in the previous story, but coincidentally, this still involves a trip to my grandparent's house. First of all, I feel regret to this day when I think of how those critters were kept and I wonder how they even survived as long as they did. One of these little Critter Keepers
served as a home for this poor creatures entire life. A sloped gravel bed to allow the newt to stay dry and probably an inch or two of water if he wanted to get wet. I remember being told that was sufficient. Newt pellets were its sole source of nutrition. And forget about any basking lamp or UVA/UVB contraption. I would rinse the gravel/change the water when I remembered.
So my sister also had a newt, same sort of situation. It was winter and in the winter we turned off the heat when we left for the weekend to save on electricity. That meant poor little newt was going to have a hard go of it. So my father decided to stick the little critter cage on the window sill to get the guy some sun. Well, he got sun. Too much sun. The water evaporated and my sister came back home to a dried out and very dead newt.
As I said, this story happened before the other. I couldn't have been older than ten, meaning this happened a good 20 years ago now. But I still cringe and feel terrible for how I took care of these guys. In my childhood I've also kept goldfish, like most of us, in less than ideal conditions.
These are all stories of neglect in their own way. I do better these days but I still feel great regret when I think of how I treated those newts and poor Goldie the Goldfish. Somehow, however, each survived many years with me and helped to nurture my love of both aquatics and small pets in general. My wife laughs (and sobs) when she finds out how much I spend on my tanks and fish, but thankfully indulges me within reason.