With that much amount of clay content down to 15ft or greater depth, there's not much worry about leaching issues once the clay soil get saturated with water. Now, how hot is on the average at your location, and when do the heat reach above 80�F (spring/summer), remain constant or get hotter, and when it drops below 80�F (fall/winter) and remain constant? It will depict the pond/lake ideal depth as well as volume of available for your fish for various scenarios.
