So, I was thinking of getting a pond... what do I need to do?

a 30 or even a 55 gallon pond is more of a tripping hazard than a good starter size for a pond. IMO
 
If you're really interested but don't want to break the bank, check craigslist in your area for a used (crack free) pre-formed pond. They can be picked up for almost a song. Sometimes you can find pondliners on sale too, just make sure they're UV resistant and MADE for ponds. If your yard is small, a 100 gallon pond only covers about 3x5 feet and is JUST deep enough to keep a couple of fish in. If you go the pond liner route, you can make it deep, with a slightly smaller footprint, but 100 gallons is really the best starting point if you plan to keep fish. Usually, the craigslist people will include the pumps you'll need for water circulation and building a DIY filter is about as easy as DIY's go (5 gallon bucket, pump, filter media, drill for the lines... done). Post pics on what you decide!
 
I have three ponds in SoCal right now. We just had a major crisis in one of them this week. They are all between 30 gal and 100 gal. And to tell you up front I wish I had started with just one larger one. The larger ones do winter better and my fish thrive all year long. (except last week) You have to pay attention to how accessable the pond will be to cats, raccoons and waterbirds. I know some beginner Koi owners who lost all their stock to herons one summer. I have dogs so I haven't had any problems in the 4 years my ponds have been up. And remember you have to have some way to change the water as goldfish especially have large bioloads. I have a small electric pump that I use to get the muck out of the bottom. But it is so rewarding when you see your first dragonfly or hear your first frogs and realize that everything is going as it should in your special little part of nature.
 
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