When my parents were getting tired of having a fish tank, they basically stopped cleaning it and let the fish die off one by one. At the end they has a 2" Upside down catfish in a 50g breeder all by itself. Because it was a pain to keep up, there were no decor just gravel. The poor guy live for quite a while in there by himself before he passed.
I guess that's why I am such a fanatic now about keeping my tank clean!