QUESTION:
What happens with the displaced air in the fish tanks?  How do they seal
it so it can travel through the different pressure levels without
exploding when it goes up into the atmosphere?  And how do the fish keep
from getting oxygen starved? 

ANSWER from Mary Williams on April 21, 1998:
The entire system is closed and sealed before being loaded in the shuttle.
At that time, any excess air is removed. A filter and pump provide air
during the flight. The module is kept inside another container that is set
up to regulate the pressure outside the tank to offset the pressure inside
the tank, so it won't explode during pressure changes.