Really good advice already given. I'll just add this website:

www.akca.org there you will find more details on the whys and hows of pond
construction and filtration. In fact, it may be more info then you ever
wanted. ;o) ~ jan

See my ponds thru the seasons and/or my filter design:
http://users.owt.com/jjspond/

~Keep 'em Defrosted~
Tri-Cities, WA Zone 7a
To e-mail see website