we had an empty pond when we moved here, we were told it was clay lined but wasnt sure if it was. we filled it and we had to keep topping it up for 1st year (hence our doubt), then after that it sealed itself. if you keep ducks, would they not eat your fish? and ducks will poo in the water and make it filthy. the old fashioned way to clay a pond, (apparently they rotated pond sites every few years but not sure why, ) was to put the clay in place then herd the cattle into empty pond and theyre feet do all the hard work for you. good luck