You can use clay to line your pond.

http://www.aquahabitat.com/techfaqs.html

"With clay, it is important to utilize a contractor who has experience
building clay lined ponds. We recently heard of a contractor who quipped
that he has made a lot of money off people who had him build clay lined
ponds. He says he makes the extra profit when he goes back in to line the
pond with a synthetic liner. What this says to us is this particular
contractor does not posses the required knowledge to build clay ponds. If he
has sufficient knowledge, he wouldn't need to reseal the ponds. Be sure your
pond contractor is qualified for the task before you hire them. We also help
clients select the best contractor.

A qualifed contractor is crucial if you intend to use commercial Bentonite
clay. Bentonite is a very effective pond sealant in the right hands. We see
way too many people try to use this material who have no concept of its
requirements. We urge everyone to seek out a contractor who has been
successful with Bentonite instead of trying to seal their own lake. Subtle
design requirements exist in order to create an efficient seal with
Bentonite. "

"gerbil" wrote in message
...
I was thinking of digging a pond on my property - say about
twenty-five feet wide and maybe six feet deep in the middle and I was
interested in having some fish. Someone told me that a "normal" pond
would not do if I wanted Koi or goldfish either one, I would need to
make it concrete or line it with something else. Why?