the reason i said to fill it with water was to check for leaks. evidently, he determined it was leaking with out filling.
the 2 reasons you want to keep tanks full of water
..if its a concrete tank, has been leaking, the bottom can be washed out beneath it, the concrete will crack when the tank is not supported, in one of its corners
a tank that has a lot of ground water surrounding it, is more of a risk of moving/floating than a tank with out saturated ground
I am glad the OP has found their problem. but in reality, their problem has just started
the tank needs replacing, the more it leaks, the more the foundation gets washed out from under it.
especially if they dig a trench to let the water AND FOUNDATION SAND drain away from tank.
this is just my opinion, of things i have seen, i have no engineering degree.