Well keeping a Goldfish
singly is like keeping a budgie singly... it may 'look happy', but only science & animal psychology can confirm or debunk that. Happiness is simply an emotion we project upon animals. I mean if you put a betta under harsh light, is it whizzing joyfully around or frantically searching for shelter?
are relatively smart fish, and unlike a ping-pong ball or a treat, another Goldfish
will provide constant stimulation and interaction for your fish. They aren't technically 'schooling fish' like tetras and rasboras, who school for safety. In the wild Goldfish
(well carps) forage and hang around in groups, and become bored and lonely without a tankmate of the same species.
I mean I have successfully kept my current goldie singly for 2 months while I searched for a quality ranchu to keep her company, but should you plan on keeping a Goldfish
singly for the rest of its life? Nope.
Also bear in mind that even in 2010 there is a lot of TERRIBLE information being produced about goldies... I was flipping through a book from 2008 which had a section stating bowls as a suitable tank for Goldfish
if filtered >_<.