I tried the old carpet underneath and was massively unimpressed. Old carpet is stiff
and weighs a ton, a real gut buster to get it in place. the cotton jute backing rots
and if the liner needs to be removed it is mold central.

OTOH, the foam underlay that goes under the carpet is fantastic. It is light as a
feather, can easily be torn or cut to fit into shape and a little duct tape holds it.
After a few years it was still "springy" underfoot. Have spent anytime in the pond
lately (having learned my lesson). It is well worth it to get NEW foam underlay,
carpet places may have ends or pieces cheap, because old foam may have staples and it
just isnt worth it to lay that expensive liner only to have it spring a leak.

Ingrid

Old carpet or that cloth pond liner underneath is the ticket.
Somewhere between zone 5 and 6 tucked along the shore of Lake Michigan
on the council grounds of the Fox, Mascouten, Potawatomi, and Winnebago