Maybe I am misunderstanding, but it appears you're using the equation for the volume of a cone........the tank is spherical.
dV equals the area of the liquid multiplied by dy.
Try to equate the area of the liquid in terms of h.
By the diagram, since the tank is a sphere, the surface area of the liquid is a circle with radius
The area of a circle is , so we have the area of the surface of the liquid at time t as
So, we have
Now, we have a separated DE.