I would use pond liner. It's the same liner and sometimes made be the same company but in roofing liner they use chemicals in it to prevent mold and stuff so it's not really fish safe. I use to work for a pond store so I know this is true. You can get pond liner that is 45mil and you could use a bottom drain. I have heard of people using the roofing liner and getting away with it but do you really want to risk it?
