Answer
16 fish
Work Step by Step
r=$\frac{1}{2}$d
r=$\frac{1}{2}$(6)
r=3
V=$\pi$r$^2$h
V=$\pi$(3$^2$)(3)
V=$\pi$(9)(3)
V=$\pi$(27)
V=27$\pi$$\approx$84 m$^3$
$\frac{84}{5}$=16.8
Therefore only 16 fish will have enough water in the tank.