I was thinking about doing a small pond & maybe putting gold fish in it. What are Koi fish? Someone I know has those in their pond. Are they commmon fish? Easy to find or buy? Are they a type of gold fish? All I know is that they get huge.
Koi are not a kind of goldfish, though that is a common misconception as they look a lot like giant goldfish. They are a kind of Japanese carp. So far as I know, they are easy enough to find...or at least the lfs I most commonly shop keeps all kinds of them. They can be quite expensive, and as you already know, they get quite large. Up to 3' is what I have read. Their average life span is 25-35 years, though some have been reported to be substantially older. A koi pond would be quite cool, and I am sure a bit of work. Do lots of research!
Koi are related to goldfish, although, as sgould says, they are not a type of goldfish.
You can find them at most pet stores, or you can find koi stores in a few places. Some will be really small, no larger than a moderate sized goldfish. Some will be really cheap, no matter how beautiful they are. Others will be unbelievably expensive, despite the fact that they just look like an oversized white and orange goldfish. The ones at the pet stores will probably run from five to thirty dollars, unless they've got some large specimens, which could reach the hundreds. I've seen koi stores whose "junk" koi cost $1,000. If your friend has koi, it's very possible that your friend will end up having koi fry soon, and might be willing to let you pick a few out.
There is a type of koi called butterfly koi that has longer fins. They're really beautiful, but because they are not a pure strain, they are considered bastards by those in the breeding scene (think of the biggest snob at a dog show). They don't grow as large as normal koi, but seem to be just as hardy.
Again, as sgould said, they get huge. They can easily live for decades. They also produce a massive amount of waste, so good filtration is a must. Like goldfish, they are very hardy, and can even survive their pond freezing as long as it is cared for and doesn't freeze too deeply. Koi are very intelligent. Mine recognizes my footsteps. They can be trained to eat from your hand, and even do tricks.
Research and planning are important. Don't forget to consider local predators. The easiest way to deal with them is make sure that there is no shallow area of the pond, and no gradual approach. Predators can't hunt from sudden drop-offs, and many fishing birds require shallow water to fish in.
Koi are a type of carp - goldfish are also a type of carp. some good and bad for both---
will continue to grow regardles of size of tank/pond
will eat any live plants you may have in your pond
beautiful colors available
can be expensive
can be trained to eat out of your fingers
require plenty of oxygen
can be sensitive to temperature changes
will grow to the size of the tank/pond they are in
not quite as likely to eat your live plants
basic orange, orange/white, white colors (altho there are shubunkins which are a mottle blue white orange color)
can be expensive also, tho if you just start with the small "feeder" goldfish you can usually find them for 25 cents or so at Petsmart.
My larger ones will come and eat from my hand
Hardier than Koi when it comes to oxygen levels/temperature.
I love the color variations you can get with Koi, but I only have goldfish in my pond. They are hardier, easier to care for, and less expensive to purchase. All of mine started out as the little 10 - 25 cent feeder fish and some are now a good 12 to 14 inches long in my 120-140 gallon preformed pond. I also have lilies - My pond gets too much sun for any of the underwater plants to do well.
Koi are a type of carp originating in Japan. Many have similar colorations to goldfish but some may include colors such as blue and yellow. They have larger eyes then goldfish and whiskers on the sides of their mouths. They are quite common and firly cheap. At PetsMart small koi are usually between 3 and 4 dollars. You don't want to get larger koi. They are more expensive and probably older. Don't be discouraged about getting smaller koi. They do grow rapidly and may outgrow the pond.