If you think that a large pond is simply a small pond that "grew up", you're in for some pond maintenance problems. Let's start off my defining the terms that we'll be using here. A large pond is anything over 1,000 gallons (5,000 liters). A pond that holds over 4,000 gallons (20,000 liters) is a very large pond.

Large ponds require a whole different level of financial and time committment than small ponds do. You'll need larger and more efficient pumps and filters as well as the additional plumbing that those devices require. You'll probably also spend a lot more for plants and fish as well as higher maintenance costs for those plants and fish.

Your building and maintenance costs are dependant, to a great extent, on how deep your pond is. A 6' deep pond, for example, calls for a large and highly-specialized biofilter which uses a lot of electricity and requires a pretty hefty chunk of space for installation. You'll need extra UV power and some sophisticated plumbing including bottom drains, skimmers, valves, and the like.

Of course, you always have the option of letting your pond go "au natural" and develop into a self-maintaining ecosystem just like a real pond. If that's your goal, don't go for a 6' depth. Natural ponds will be more cloudy than a filtered pond and it's likely that you'll never see your fish!

Even a natural pond will require some help from you before mother nature kicks in. You'll still need to condition the water, and remove chlorine, before fish are introduced. You'll also want to have lots of plants both for natural shelter and temperature regulation as well as for their oxygen-generating characteristics.

If your pond takes root and begins to develop naturally then you'll actually end up saving quite a bit of money on fish food. A natural pond generates and attracts a lot of natural food sources and it's likely that you will never have to feed your fish at all.

Don't skimp on testing just because your pond is developing naturally. You still have threats of fertilizer and pesticide runoff entering your pond, and you still need to worry about keeping oxygen levels up and water temperatures down. You'll also want to examine your fish periodically for signs of parasite infestation.

Keeping a large pond can require an inordinate amount of time and money. My best advice is not to build one unless you are willing to do your part in keeping it.