For an alternative to a hard plastic fish tank, talk to Zeke at Billboardtarps. Pretty inexpensive used as a pond liner (supported of course). They are fish and human friendly according to Zeke. See the website at: http://www.billboardtarps.com/collections/tarps.

If anyone knows of a problem using this method, please let me know, but I've seen it used. Tell him Gary sent you
(no, I don't get a commission )

ArtfulDodger