A damaged swim bladder.
When they begin to swim on their side, they are usually "on the way out", as the saying goes.
What else would you expect them to swim in? Of course Goldfish swim in water.
Swim bladder disease? http://www.straightdope.com/columns/read/1688/if-a-goldfish-is-swimming-on-its-side-is-this-a-bad-sign
No
When their swimming upside down its usually a sign that their dying
because they are fish
NO. Unless there in ranch
they don't notice the blockage so they think they can swim right through
Anywhere :)
Freshwater
sometimes if there in the mood
Swim, Eat, Swim, Eat, Poop, Swim, Eat,Poop....