i'm in the process of building my pond "planning stage" i would like it to be 65 foot wide and 50 long and about 4 feet deep. its small but it is in the center of my circle driveway.my wife wants a rock waterfall.my question is should i use a clay liner or a eprm liner. i have a dug test holes and found no clay i have cap rock about 18" below surface. a liner would cost about 1,000. but then would have to filter where a clay liner would be more beneficial to the fish and require less filtration.also how do i determine the amount of clay to use. thanks