Use the arrow keys to move around, up arrow to counteract the gravity. Pushing against the top or the bottom of the screen will build up movement past the edge, making it seem stuck for a while. This will not be a problem when all the walls cruelly murder you.

Haha thanks! :D The fish are actually re-colored x-rays of fish that I modified and shrunk to fit the game, and I agree, the predators look pretty weird. And yeah I'll see if I can do that, thanks. :)