Koi are so much fun to feed, they are true hogs. The only regrets often are many would build it differently the second time around. Following are some of the regrets:
First and most importantly make it as big as you can. Most of us start out with a pond from 500 to 2,000 gallons, and then build another one that is 10,000 to 50,000 gallons. The reason is we want more and more Koi. There are hundreds of combinations of colors and varieties, and we want one of each. We can't put more than about 20 full size Koi in a 2,000 gallon pond, and to do that requires a lot of filtration and aeration.
Second, do not put any goldfish into your pond. Many of us spend our lives trying to get the gold fish out of there because they breed so easily.
Third, make it at least 4' deep; 4' keeps out a lot of raccoons who love the taste of Koi; 8 feet doubles the size. 8' means you can't stand up in it without taking out a lot of the water, but it will make it a little more difficult for Great Blue Herons, Kingfishers, etc to clean out your pond.
Fourth, just in case, have a geometry that will allow you to put a bird net over the pond. A single Great Blue Heron can eat a hundred or more Koi (6") in just one meal. They are a very serious threat, and they can cost you thousands of dollars, particularly during May and June (in Colorado) when they are feeding their young. Sometimes they will just kill a 2 foot long Koi, and leave it around your pond.
Fifth, locate the pond out of direct sunlight all day long. Koi need shade from direct sunlight; you can provide plants (like water lilies) with plant stands, or clay drain tiles, anything that will give them relief. Keep it away from overhanging trees if possible so the leaves and needles don't blow in. However, sometimes we just have to deal with an imperfect location.
Sixth, you need a properly sized water pump, filtration system, and an ultraviolet sterilizer. We can help you with those, and we believe we are more competitive than anyone else. In fact, I don't know of anyone in the country that can beat our prices. If your budget won't allow a properly designed filtration system, you can limp along on homemade stuff for awhile, until you can afford it. We have directions for homemade filters on the website, but all the money you sink into the homemade stuff will be wasted when you build a proper system. More importantly, you will work much harder and longer trying to clean out the filters, etc., than you will with a proper system.
Seventh, an inexpensive cattle trough heater will keep a hole in the ice during the winter hard freezes. They are available in 1,000 and 1,500 watts. Moving water or aeration will also keep it from freezing. You want to avoid a solid ice cap over the entire pond.
Eighth, if you have the money we recommend a gas pond heater for cold climates. Koi don't eat when the water is much colder than 50 degrees, and they don't grow when they don't eat. A pond heater will greatly lengthen the growing season, which means they will get bigger faster. It will also reduce the fluctuations in temperature between a hot day and a cold night.
Ninth, very few ponds have a protein skimmer, but they are very useful to reduce the organics causing the foam under your waterfall, etc.
Tenth, be ready to easily replace 5-10% of the water every week to keep the water fresh, and to reduce the buildup of organics. Of course that may require replacement of some of the chemicals more often.
Eleventh, never forget that chlorinated water will kill your Koi. You need to buffer the pH, and continuously monitor the ammonia and nitrite (that's different from nitrate) levels. They can kill your Koi, and will also tell you about over feeding and over crowding. It is also important to test the chlorine, pH, alkalinity, hardness, salinity, nitrate, phosphate, copper, and oxygen.
