I would set this up as an integral over the distance from the top of the tank to the water level. If we consider an infinitesimal circular "slice" of water whose surface is a distance y from the top of the tank, the radius of that circular slice is:
The volume then is:
(We approximate the volume of the cone using a cylinder of height and take the limit as this height goes to 0.)
Integrate from to the equation for the amount of work required to move such a "slice" to the desired height . (observe that work depends not only on the volume but also on how far the water has to move which is this case is ).