Our fish pond is pretty big...a kind of kidney shaped one that I dug and lined myself.
I'd estimate about 900 gallons. (It's about 15' x 4' x 2 1//2' in depth.)
Got mostly goldfish...decent sized; some shubukin. We had some koi, but they were not able to withstand the winters here. I keep a heater in the pond during the winter...but that just keeps a melted hole for some oxygen intake and doesn't actually warm the water. The goldfish, strangely enough...last for years and years. Some of ours are 15 years old. Beautiful, huge tails.
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Give it a click.