hi all. i am installing a used 18ft pool and am having trouble with leveling. everything says it is level but after i add about a foot of water my liner comes tumbling down. i thought maybe because i put the 6 inch birm around the inside bottom wrong. can anyone tell me how it is supposed to be put down? and how level is the ground supposed to be anyway. the film says within an inch, but it doesn't seem to be working. have dumped tons of water now as it has tumbled three times to this point. am not sure what to change next, lol. thanks for any in put.