Hi Warr/Reaper/Wee-Man -
I got a VERY interesting marine aquarium Q & A book (no - NOT from Calfo - there are MANY other knowledgeable people who actually have done doctorates on marine biology) which explains fish behaviour very much in-depth....
This book says that fish actually has a built-in clock, as well as direction senor (based upon light direction, light strength/intensity, coloration, etc) that they use to determine which side is up or down, and what season it is, when it is time to breed, what the tides are, etc....
(And no - it is not their swimming bladder Reaper).
This also means that IF light would shine from one side of the tank, for longer than a certain period, they would indeed most likely swim in the way that they "think" is "up" (where the light is shining from).
This is the exact same way that fish which usually swims in caves swim upside down, because it is darker on the cave-roof, then where the sand is. The fish then think that the direction where the sand is, is "up" and the direction where the "darker" (cave roof) area is, must be down....
Even on a moonless might, there is still light shining down on the waters over the reefs.... The reefs are nearly ALWAYS lit from the top (when would the reefs NOT be lit from the top?). BUT - there are times that fish swim into caves and become "dioriented". Like a SCUBA diver can become disoriented in open water (not knowing which side is up and which side is down), without visual clues determining this?