I used to wonder about this. Almost every time I saw goldfish in an aquarium, with quality filter and food and everything, some fish still ended up dying early, or getting freaky-looking illnesses and tumors like:
The other day I found out, these fish are in fact sensitive to the SIZE of their aquarium as well!
Even though goldfish have a short term memory like a lot of small fish do, and being in a small home rarely makes them panic, it will still damage them with stress. (EDIT: Just got informed that goldfish have a memory span of 3 months, my bad. Still not so much memory span compared to the fact they can get around 40 years old, though.)
It reminds me of the cases of PTSD, where people forget cruelties because of mental self-defense, but they still live with a noticeable damage.
And then I also found out, the size of the home will not only influence the health and life span, but also the SIZE of a goldfish! And I mean, a DRAMATIC change of size.
Look at what becomes of goldfish when they're released into a larger (and clean) environment:
I thought about all this for a while. I get a spiritual inspiration from this.
We all finally need to free ourselves from the small aquarium we live in, so we can be truly healthy and grow like we should. :U