I like Koi and goldfish, always have.  So, I snagged a free hot-tub shell from craigslist.  It's 7' x 7' , and I estimate it weighs somewhere around three hundred pounds.
hot-tub shell 2x4 frame
diy koi pond spa pond
custom paint job

For the filter I scavenged / dumpster dove a dishwasher pump, and mated it to the existing 1 1/2 inch pipe for the tub.  First, I tried putting the filter on the intake, but it clogged to easily.  Then I went for a biological style filter, made out of a 5 gallon bucket, some sand, and a pillow (the poly-fil stuffing was cheaper in a pillow than in a bag)
diy pond filter