Why do fish swim upright?

While running a design session this week I was suddenly struck by a strange thought.

Since fish live in a completely 3-dimensional environment and typically spend their time in a state of weightlessness (neutral buoyancy)…why do they swim upright (e.g. with their bellies facing towards Earth)?  

I posed this question to the team (a bit of a diversion from designing software I’ll admit) and, despite all being intelligent people with top-notch educations, none of them could come up with a plausible answer.

And then, while making some toast yesterday, another thought occurred to me.  Since I understand why land animals stay upright, perhaps the current thinking about life starting in water is incorrect and things actually started on land.  And would that mean that those creationist people actually have something right after all?

These were odd thoughts for several reasons:

1. Well…they are just strange thoughts anyway.
2. I normally get my strange thoughts in the shower.   (Hey!  You!  Get your mind out of the gutter)

Inquiring minds want to know…

Answers in comments below please.  A hearty handshake and a gold star to the first person with a plausible answer.