Posted: 10/05/2013 at 18:37
And you need a good depth of mud in the bottom for the frogs to snuggle down into.
Was it an old cast iron bath - I imagine that it would've got very cold indeed. As Nutcutlet says, you'll do much better with a pond liner, and the pond should be roundish in shape, rather than long and thin like a cast iron bath, then there's a chance that the deep water middle won't freeze as quickly down at the bottom.
And a bog garden at the side of the pond will attract no end of wildlife - good luck