Swim bladder disease?
http://www.straightdope.com/columns/read/1688/if-a-goldfish-is-swimming-on-its-side-is-this-a-bad-sign
If the goldfish is swimming in water, it is alive.
Yes
it stops swimming
No!
Gravity
if it is not swimming and it lays on its side then it is dead.
It's not swimming. It's dead. Sorry :(
Stop doing a handstand. If your goldfish is floating, it will float to the top, not the bottom, therefore, you must be upside down!
The fins develop on a goldfish fry after about a couple days after they are hatched. Until then, they are not free swimming and look like a tadpole.
Probably because its water is foul and needs changing
Cause They Like Swimming Side Ways
Most goldfish bowls, aquaria, swimming pools, seas, oceans...