Hello I'm building an above ground pond on a cement slab under a covered carport. I'm using railroad ties for the sides. I have currently stacked the railroad ties 2 high. When finished the water depth will only be around nine inches. Do I need to make it deeper to support fish? I'm at a stage where I could add another course of ties which would increase the depth to about 12-13 inches. I also need to know how to heat the water during winter to keep the fish alive. I live in Paducah, Kentucky, which I believe is zone 6. Any suggestions would be greatly appreciated.