That is roofing iron around the bottom, there is rocks just below the surface up the waterfall end of the pond, i was really worried about the Blueys at first so i put in the rocks and gave them all a test swim, most people don't believe it but they love it, i was digging out some eggs from under the pond one day had my head right under the lip , when i look up heres one of the Blues she's swum over to check out whats going on head covered in duckweed looking down at me.
I put the 2 smaller ponds in the waterfall so they could drink without getting into the big one but you constantly see them in the big one or lying around with duckweed all over them, i'll put up a couple of pics i took today the pond has changed a bit.
Cheers
Nick