The smalles possible inside length will be the one that gives exactly the volume needed, anything less won't hold the required volume of water.
Since it is cubical, all sides have the same length and the volume will be:
Let's first convert L to m³. 1 L is equivalent to 0.001 m³, so 180 L is equivalent to
Now, we input that volume into the fisrt equation and solve for "l":
So, the smallest possible inside length of the tank is approximately 0.565 m.