If there is excess pressure (height) in the tank, one would be better off to lower the tank and save the power in pumping.
The potential energy in stored water is M*g*h where M is the mass, g is the acceleration of gravity, approximately 9.8 m/s^2, and h is the height. If one used Kg for the mass, and meters for the height, then the result of multiplying out the equation will be in Kg*m^2/s^2 which is the definition of Joules, a unit of energy. A Watt is equivelant to one Joule per second. One KWH is equal to 3,600,000 Joules.
You can do the math for your situation, and see the energy available in your tank.
As Ron has correctly pointed out in his own special way, considering the effort involved to generate power this way, one would generally be looking for a greater height differential, and a greater mass of water to make such a project more practical.
Double check my math, but if one had even 100 meters of height differential, one would have about a KWH of stored energy per every 970.4 gallons of water. If you recover 75% of that you would need about 1294 gallons of water per KWH.
Make your project work for you - Rich Hagen