Taking care of your Koi pond and keeping it in good condition should be your primary priority. Always remember that prevention is better than cure: once your pond starts to develop problems, it'll take more time and money to fix these.
No overfeeding:
Overfeeding your pets can cause damage to your water quality. Your fish wouldn't nearly eat the amount of food that they do in a controlled environment. Feeding them with too much food will produce a larger amount of waste in the pond. What you should do is to vacuum the bottom out as much as you can, and also make sure that the drainage systems are clear and working properly.
Keeping the water quality:
Your pond needs a filtration system. There are two types of this system. The mechanical filtration is the one that relieves the pond of dead algae, wastes and insects (this is important to sustain the size of your pond and the size and number of your pets). The biological filtration is the one that provides the nitrogen cycle, removing dissolved wastes from your pond (without this, ammonia will build up which is very devastating to your fish).
Keep your pond clean so that your fish will live a beautiful and stress-free life. The water in your pond is where they will eat, sleep and feed, so it's very important to keep it as clean as possible. Your pond's water should be almost completely clear. It should contain no debris, and your fish should appear like they're floating in the air. Also, you should be able to see very well the bottom of your pond.
Keeping supplies on hand:
There are a lot of supplies that you need to properly maintain your pond, and the majority of these items are of great importance. You should know the function of each item to understand why they're needed. There are also items that you need to have on hand all the time. As your pond is located in the outdoor, emergency situations are sure to happen. We all know that nature itself is at times unpredictable, so don't get caught in a situation where an item is needed and you have no access to get it!
Preventing the common illnesses:
If you have a newly-bought fish, it is very important to quarantine them from your current fish population. Do this for no less than 15 days. Knowing that KHV (or Koi Herpes Virus) is activated within temperatures at 74 degrees Fahrenheit will let you expose your pet to the right conditions for the disease to reveal itself. Those that live in these conditions for a longer period of time and don't have any of the symptoms will show that they have little chances of contracting the illness.
As you quarantine your fish, keep it in complete isolation, away from your current population. Separate the tools, food and water that you use to take care for your "newbie". Proper hand washing procedures should be followed as well in handling both the Koi and Koi items within the same period of time. Keep in mind that quarantining will greatly reduce the chances of exposing your pets to viruses and diseases.