Answer
$\frac{2}{27}$ of expanded body volume
Work Step by Step
The fish needs to inflate its air sacs such that its density is $1$ $g / {cm}^3$. As it does this, its mass will stay constant but the volume will increase. 1.08-1.00 is 0.08 and $0.08/1.08 = 2/27$= $\frac{2}{27}$, so the fish needs to inflate the air sacs to that fraction of its body volume to reduce its density to that of water.
