Since we keep our koi outside in a medium to large sized pond, we'll mainly be covering how to care for koi in this type of environment. In our opinion, an outdoor pond (or traditional japanese style garden) is the greatest setting to truly appreciate koi the way they have classically been viewed for hundreds of years.
Some topics we'll be covering: (click topic to go)
General Maintenance:
Koi are very hardy fish. In other words, it takes alot to kill them. With that being said, there are only a few things that should be done on a daily bases to keep your koi alive. Any debris that falls into the pond should be removed. This can include leaves, pinecones, sand, dirt, uneaten food, etc. For us, pine needles are a real problem. We bought a swimming pool net to help us scoop the needles out of the pond, and its works quite well.
You will also want to keep any aquatic or semi aquatic plants well trimmed and maintained. This will help to prevent unnecessary debris from entering the pond. In the summer, algae will often spread wildly throughout an outdoor pond. Personally, we don't like to use algaecides, so we just scoop any algae that grows every 2-3 days.
Filtration Systems:
For keeping koi, or any fish (besides maybe goldfish) for that matter, a adequate filtration system is required. The filtration system should have mechanical and biological filtration aspects, and should be rated correctly for the size of your pond. Filters can be set up in a number of different ways. Ours has an intake, which takes water from the pond through the actual filter. Once filtered, the water is pushed through to the top of a water fall, where is falls back into the pond as clean water. Its hard to say which filtration system is good for you. The best bet is to talk to your local koi shop (or fish store).
Water Quality:
Water is to fish, as air is to humans. If that air is smoky, of smoggy, or smelly, or whatever, we don't do very well. We start to cough, sneeze, etc. It is extremely important to keep your koi's water clean. This can be accomplished by using the correct type of filtration system, and performing daily pond maintenance. Water quality is a science, anyone who has ever successfully kept a nice aquarium will tell you this. A pond in no different. Every koi owner should have a water quality tester kit. These kits will tell you various different levels of your pond. Things you should look out for are chorine, and ammonia.
You should change approximately 20% of the water in your pond every month. If you are using a standard hose from your house, it will probably have small amounts of chlorine in the water. Make sure to supplement the water with the proper chlorine remover. It is also wise to let the water spray into the pond, rather than having the end of the hose submerged. This allows for aeration for dissipation of the chlorine gas. Click here for more information of water quality.
Feeding:
Just like any fish, koi will eat every bit of food you throw in the pond. And just like any fish, over feeding them is bad, and can cause death. There are different kinds of koi food for practically every season of the year. Koi should be eating much more in the spring and summer than they do in the fall and winter, and should be fed accordingly. Feeding once, maybe twice a day is fine.
One of the great things about koi is that you can "train" them to eat of of your hand. It takes lots of time and patience.
Predatory Animals:
Because of the construction of koi ponds (shallow water), the koi within are often the prey of predatory animals. Common animals to watch out for would be raccoons, and predatory birds such as, hawks, eagles, and herrings.
The best way to keep you koi safe from animals like raccoons is to built a small perimeter fence around the entire pond. It doesn't have to be more than a foot tall, just enough to keep the animal from being able to sit and reach into the pond. Birds are harder to deter. The best thing would be to have some kind of netting over top of the pond, but this would be ugly. Our pond is underneath a couple of trees, which effectively blocks the view of any high flying predator.
It also helps to initially dig your pond a little deeper so the koi have a place to hide. Include aquatic plants, as well as rocks and overhangs.