I too have kept fish in them no problems at all. If you are that concerned about it you could always use liner in it. I really don't see the need for that though. Kind of defeats the purpose.
Rubbermaid makes stock tanks that are made for holding water for livestock to drink. Many people have kept fish and turtles in them without incident (myself included)
Our neighboor used to keep goldies in his horse tank.
They survived 2 years without being fed as well after his passing in an above ground unheated tank, and winters here get to around 5 on a few nights, but mostly around 25.