Koi and goldfish? There's a difference?
Koi? What are koi and why must I build a koi pond differently than any other pond? Can't I just put koi in my pond? One would think so, but one would be wrong.
First of all, a koi is a carp, just like a goldfish, but it is from a different family. Goldfish are descendants of crucian carp and koi are from common carp. Koi usually have two whiskers like a catfish and goldfish don't, so they are easy to spot if someone wants to give you one.
And there are other differences as well. Goldfish dart around more in the water and will eat your submerged vegetation as they fertilize it. Your pond will be a relatively maintenance free ecosystem with plants and goldfish. Koi, on the other hand, swim lazily around eating everything they can get their mouths around. They love your most expensive water lilies and will dispatch with them first. They work their way through every piece of vegetation you have in your pond and look for more before you even realize you must feed them.
So a koi pond is a special outdoor aquarium made just for koi. Koi will pull plants down from outside the pond just for sport. They also have a awful habit of jumping out of the pond where you find them stiff and dry when you come home from work. The jumping out is often a sign of foul water. A goldfish pond is a water garden with goldfish swimming around behaving themselves.
Before you even think of building any pond, think long and hard: Do you want a koi pond or do you want a water feature with plants and goldfish? For goldfish pond building see How to Build a Pond
You have decided you want a koi pond because you want koi as pets.
A koi pond must be deeper than a goldfish pond. Koi need more room to move around. They like to swim up and down as well as back and forth. They also grow and grow fast, so make your pond at least 3' deep, deeper if you can and as big as you can afford. Try to get your koi pond dug below the frost line or you will be trying to figure out how to over winter them in the house when it freezes outside.
A koi pond should be built up above the ground. I like to do this with goldfish ponds as well to keep run off out and therefore avoid chemicals that may run into your pond. If you can get your koi pond edges up at least 6", you will be safer and possible keep your koi in the water instead of lying on the ground. I have known professional koi keepers who build their ponds at least 18" above ground, usually using concrete for the entire pond rather than a butyl rubber or EPDM liner.
Koi ponds must have filtration. As much filtration as you can afford. Do you get the idea that koi keeping can be expensive? In the past koi keepers used swimming pool filters. Now bead filters are popular. If the bead filters are just too expensive, a quality biofilter will do. It should be big enough for your pond. Most biofilter manufacturers will help you pick out the one that will work best for your pond size.
Using a UV light sterilizer, usually called a water clarifier is necessary for your koi pond. It will kill algae microorganisms and keep the water clear so you can see your fish.
Koi are hungry fishies. You must feed them. You will find scores of koi foods available. I make no recommendations about which one to buy. I suggest finding a koi club in your area and see which ones their fish like and following their examples and suggestions. One thing I know koi love to do is play with a half of a red cabbage. Don't shred it. Let them play with it like a basketball.
The water chemistry must be perfect. Any radical changes in pH, nitrate, nitrite, ammonia must be corrected immediately or your fish will get sick.
The biggest difference
Don't forget one basic difference. When you build a koi pond, you are building a special house for your pets. When you build a goldfish pond, you are putting a water garden in your landscape.
And you will name those koi, I know that. Because you named your children and your other pets. Koi can get sick and die of the strangest diseases before you are even aware they are sick. I suggest not naming them.